diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs
index 33eafca8..e62c8d2d 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DriverHostActor.cs
@@ -1793,9 +1793,10 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers, IWithUnboundedS
         // #626 — a QUALITY-ONLY annotation (an MTConnect condition gone Unavailable, or back) is not an alarm
         // transition. It rides the SAME quality-only node path the #477 connectivity annotation uses
         // (AlarmQualityUpdate → WriteAlarmQuality, which touches Quality and nothing else), never a projected
-        // snapshot: the projector's prior state is cleared on every apply, so a projected "keep the state"
-        // after a deploy would default to inactive+acked and silently acknowledge a retained, unacked
-        // condition — and would rewrite its message and severity. Ungated by role (a Secondary keeps quality
+        // snapshot: the projector never sees a client's acknowledgement (that is applied to the node by the
+        // SDK) and forgets a condition a deploy removed, so a projected "keep the state" can disagree with
+        // the node — silently acknowledging a retained, unacked condition or rewriting its message and
+        // severity. Ungated by role (a Secondary keeps quality
         // warm, as for connectivity) and no alerts row.
         if (msg.Args is { IsQualityOnly: true, QualityStatusCode: { } quality })
         {
@@ -3078,7 +3079,8 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers, IWithUnboundedS
         _alarmNodeIdByDriverRef.Clear();
         _driverRefByAlarmNodeId.Clear();
         _alarmMetaByNodeId.Clear();
-        _nativeAlarmProjector.Clear();
+        // NOT _nativeAlarmProjector.Clear() (Gitea #641): it is pruned to the still-deployed conditions after the
+        // maps are rebuilt below, so a redeploy that keeps an alarm keeps its projected prior state.
         // The per-driver secondary indexes are rebuilt WHOLE alongside their primaries below — same pass,
         // same clear — so no incremental maintenance exists to get out of step with them.
         _valueRefsByDriver.Clear();
@@ -3132,6 +3134,13 @@ public sealed class DriverHostActor : ReceiveActor, IWithTimers, IWithUnboundedS
             }
         }
 
+        // Gitea #641 — forget the projected prior state of conditions this composition no longer carries, and keep
+        // the rest. Clearing it whole on every apply made the next driver Acknowledge of a still-active alarm
+        // project Active=false (the default prior), and the next Clear of an unacknowledged one project
+        // Acked=true — on any deploy, rebuild or not. The node manager carries the NODE's state across a rebuild,
+        // so the two stay in step. Keyed by the bare condition NodeId, exactly as ForwardNativeAlarm projects.
+        _nativeAlarmProjector.RetainOnly(_driverRefByAlarmNodeId.Keys.ToHashSet(StringComparer.Ordinal));
+
         // Cache the per-driver payload as THE authority, in the same clear-and-repopulate pass as the routing
         // maps above so a rename/removal can never leave a stale entry behind. Keyed over the UNION of the two
         // ref maps AND the running children: a child with no authored tags must get an explicit EMPTY set (which
diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/NativeAlarmProjector.cs b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/NativeAlarmProjector.cs
index 4e52cf90..fb0f8075 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/NativeAlarmProjector.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/NativeAlarmProjector.cs
@@ -40,9 +40,25 @@ public sealed class NativeAlarmProjector
             StatusCode: e.QualityStatusCode ?? OpcUaStatus.Good);
     }
 
-    /// <summary>Clears all tracked per-node state (call on address-space rebuild).</summary>
+    /// <summary>Clears all tracked per-node state.</summary>
     public void Clear() => _prior.Clear();
 
+    /// <summary>
+    ///     Gitea #641 — drop the tracked state of every condition NOT in <paramref name="deployedNodeIds"/>, keeping
+    ///     the rest. Called on every apply instead of <see cref="Clear"/>: a condition that is still deployed keeps
+    ///     its state across the deploy (the node manager carries the node's state across a rebuild too), so the
+    ///     next Acknowledge does not project <c>Active = false</c> onto an active alarm and the next Clear does not
+    ///     project <c>Acked = true</c> onto an unacknowledged one. A removed condition is forgotten, so a later
+    ///     re-add starts from the default.
+    /// </summary>
+    /// <param name="deployedNodeIds">The condition node ids the new composition still materialises.</param>
+    public void RetainOnly(IReadOnlySet<string> deployedNodeIds)
+    {
+        ArgumentNullException.ThrowIfNull(deployedNodeIds);
+        foreach (var nodeId in _prior.Keys.Where(k => !deployedNodeIds.Contains(k)).ToList())
+            _prior.Remove(nodeId);
+    }
+
     private static ushort MapSeverity(AlarmSeverity s) => s switch
     {
         AlarmSeverity.Low => 200,
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverHostActorNativeAlarmTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverHostActorNativeAlarmTests.cs
index 848ee47d..2a2fadaa 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverHostActorNativeAlarmTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/DriverHostActorNativeAlarmTests.cs
@@ -409,6 +409,66 @@ public sealed class DriverHostActorNativeAlarmTests : RuntimeActorTestBase
         publish.ExpectNoMsg(TimeSpan.FromMilliseconds(500));
     }
 
+    /// <summary>
+    ///     Gitea #641 — a redeploy that keeps the alarm must not reset the projector's prior state for it. It used
+    ///     to be cleared on EVERY apply, so a driver Acknowledge arriving after any deploy projected
+    ///     <c>(prev.Active = false, Acked)</c> and wrote an ACTIVE alarm inactive.
+    /// </summary>
+    [Fact]
+    public void An_acknowledge_after_a_redeploy_keeps_the_alarm_active()
+    {
+        var db = NewInMemoryDbFactory();
+        var deploymentId = SeedV3AlarmDeployment(db, RevA, Driver: "drv-1", Tag: "temp_hi");
+        var (actor, publish, coordinator) = SpawnHostAndApplyWithCoordinator(db, deploymentId);
+
+        actor.Tell(Transition(AlarmTransitionKind.Raise));
+        publish.ExpectMsg<OpcUaPublishActor.AlarmStateUpdate>(TimeSpan.FromSeconds(5)).State.Active.ShouldBeTrue();
+
+        Redeploy(db, actor, publish, coordinator);
+
+        actor.Tell(Transition(AlarmTransitionKind.Acknowledge));
+        var acked = publish.FishForMessage<OpcUaPublishActor.AlarmStateUpdate>(_ => true, TimeSpan.FromSeconds(5));
+        acked.State.Active.ShouldBeTrue("the alarm is still active at the device");
+        acked.State.Acknowledged.ShouldBeTrue();
+    }
+
+    /// <summary>Gitea #641 — likewise a Clear after a redeploy must not silently acknowledge an unacknowledged
+    /// alarm (the cleared prior state defaulted Acked to true).</summary>
+    [Fact]
+    public void A_clear_after_a_redeploy_keeps_the_alarm_unacknowledged()
+    {
+        var db = NewInMemoryDbFactory();
+        var deploymentId = SeedV3AlarmDeployment(db, RevA, Driver: "drv-1", Tag: "temp_hi");
+        var (actor, publish, coordinator) = SpawnHostAndApplyWithCoordinator(db, deploymentId);
+
+        actor.Tell(Transition(AlarmTransitionKind.Raise));
+        publish.ExpectMsg<OpcUaPublishActor.AlarmStateUpdate>(TimeSpan.FromSeconds(5)).State.Acknowledged.ShouldBeFalse();
+
+        Redeploy(db, actor, publish, coordinator);
+
+        actor.Tell(Transition(AlarmTransitionKind.Clear));
+        var cleared = publish.FishForMessage<OpcUaPublishActor.AlarmStateUpdate>(_ => true, TimeSpan.FromSeconds(5));
+        cleared.State.Active.ShouldBeFalse();
+        cleared.State.Acknowledged.ShouldBeFalse("nobody acknowledged it");
+    }
+
+    private static DriverInstanceActor.AttributeAlarmPublished Transition(AlarmTransitionKind kind) =>
+        new("drv-1", new AlarmEventArgs(
+            new StubAlarmHandle(), "Temp", AlarmRawPath, "OffNormalAlarm", kind.ToString(), AlarmSeverity.High, Ts,
+            Kind: kind));
+
+    /// <summary>A second deployment carrying the SAME alarm (a new revision of an identical artifact).</summary>
+    private void Redeploy(
+        IDbContextFactory<OtOpcUaConfigDbContext> db, IActorRef actor, TestProbe publish, TestProbe coordinator)
+    {
+        var revB = RevisionHash.Parse(new string('b', 64));
+        var second = SeedV3AlarmDeployment(db, revB, Driver: "drv-1", Tag: "temp_hi");
+        actor.Tell(new DispatchDeployment(second, revB, CorrelationId.NewId()));
+        coordinator.FishForMessage<ApplyAck>(a => a.DeploymentId == second, TimeSpan.FromSeconds(5))
+            .Outcome.ShouldBe(ApplyAckOutcome.Applied);
+        publish.FishForMessage<OpcUaPublishActor.RebuildAddressSpace>(_ => true, TimeSpan.FromSeconds(5));
+    }
+
     /// <summary>Subscribe <paramref name="probe"/> to the <c>alerts</c> DPS topic and wait for the ack.</summary>
     private void SubscribeToAlerts(TestProbe probe)
     {
@@ -439,6 +499,13 @@ public sealed class DriverHostActorNativeAlarmTests : RuntimeActorTestBase
     /// publishes an alarm. A VirtualTag-host probe is injected so the real host isn't spawned.</summary>
     private (IActorRef Actor, TestProbe Publish) SpawnHostAndApply(
         IDbContextFactory<OtOpcUaConfigDbContext> db, DeploymentId deploymentId)
+    {
+        var (actor, publish, _) = SpawnHostAndApplyWithCoordinator(db, deploymentId);
+        return (actor, publish);
+    }
+
+    private (IActorRef Actor, TestProbe Publish, TestProbe Coordinator) SpawnHostAndApplyWithCoordinator(
+        IDbContextFactory<OtOpcUaConfigDbContext> db, DeploymentId deploymentId)
     {
         var coordinator = CreateTestProbe();
         var publish = CreateTestProbe();
@@ -460,7 +527,7 @@ public sealed class DriverHostActorNativeAlarmTests : RuntimeActorTestBase
         // ExpectMsg<AlarmStateUpdate> assertions see only alarm updates.
         publish.ExpectMsg<OpcUaPublishActor.RebuildAddressSpace>(TimeSpan.FromSeconds(5));
 
-        return (actor, publish);
+        return (actor, publish, coordinator);
     }
 
     /// <summary>
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/NativeAlarmProjectorTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/NativeAlarmProjectorTests.cs
index eaee1d77..e3a2c876 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/NativeAlarmProjectorTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Drivers/NativeAlarmProjectorTests.cs
@@ -24,6 +24,21 @@ public class NativeAlarmProjectorTests
         snap.Message.ShouldBe("boom");
     }
 
+    /// <summary>Gitea #641 — <c>RetainOnly</c> keeps a still-deployed condition's prior state and forgets a removed
+    /// one's (which then starts from the default inactive + acked).</summary>
+    [Fact]
+    public void RetainOnly_keeps_deployed_conditions_and_forgets_removed_ones()
+    {
+        var sut = new NativeAlarmProjector();
+        sut.Project("kept", Evt(AlarmTransitionKind.Raise));
+        sut.Project("removed", Evt(AlarmTransitionKind.Raise));
+
+        sut.RetainOnly(new HashSet<string>(StringComparer.Ordinal) { "kept" });
+
+        sut.Project("kept", Evt(AlarmTransitionKind.Acknowledge)).Active.ShouldBeTrue();
+        sut.Project("removed", Evt(AlarmTransitionKind.Acknowledge)).Active.ShouldBeFalse();
+    }
+
     [Fact]
     public void Retrigger_projects_active_unacked()
     {