diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/ContinuousHistorizationRecorderTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/ContinuousHistorizationRecorderTests.cs
index 7cccf27d..712db0dc 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/ContinuousHistorizationRecorderTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Historian/ContinuousHistorizationRecorderTests.cs
@@ -434,8 +434,13 @@ public sealed class ContinuousHistorizationRecorderTests : TestKit
             drainInterval: TimeSpan.FromMilliseconds(20),
             maxWriteAttempts: maxAttempts));
 
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow));
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow));
+        // ONE burst, so both entries share a drain pass (the #538 discriminator needs the sibling in the same pass):
+        // two separate Tells race the off-mailbox append loop, which may append and drain the first alone.
+        rec.Tell(new VirtualTagActor.DependencyValueBatch(new[]
+        {
+            new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow),
+            new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow),
+        }));
 
         // The head advances: both entries leave the outbox — the good one written, the bad one discarded.
         await AwaitAssertAsync(async () =>
@@ -519,8 +524,13 @@ public sealed class ContinuousHistorizationRecorderTests : TestKit
             drainInterval: TimeSpan.FromMilliseconds(20),
             maxWriteAttempts: 50));   // a budget the test will not exhaust
 
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow));
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Flaky.Tag", 2.0, DateTime.UtcNow));
+        // ONE burst, so both entries share a drain pass (the #538 discriminator needs the sibling in the same pass):
+        // two separate Tells race the off-mailbox append loop, which may append and drain the first alone.
+        rec.Tell(new VirtualTagActor.DependencyValueBatch(new[]
+        {
+            new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow),
+            new VirtualTagActor.DependencyValueChanged("Flaky.Tag", 2.0, DateTime.UtcNow),
+        }));
 
         // It has genuinely failed a few times before recovering — otherwise this proves nothing.
         await AwaitAssertAsync(
@@ -589,8 +599,13 @@ public sealed class ContinuousHistorizationRecorderTests : TestKit
             // this test is only meaningful when it is true.
             drainConcurrency: 1));
 
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow));
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow));
+        // ONE burst, so both entries share a drain pass (the #538 discriminator needs the sibling in the same pass):
+        // two separate Tells race the off-mailbox append loop, which may append and drain the first alone.
+        rec.Tell(new VirtualTagActor.DependencyValueBatch(new[]
+        {
+            new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow),
+            new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow),
+        }));
 
         // Presence waits, budgeted from the shared RuntimeActorTestBase constant rather than 10 s: this
         // one timed out on a loaded Gitea runner (#606). They return the instant the writer signals.
@@ -659,8 +674,13 @@ public sealed class ContinuousHistorizationRecorderTests : TestKit
             drainInterval: TimeSpan.FromMilliseconds(20),
             maxWriteAttempts: 2));
 
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow));
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow));
+        // ONE burst, so both entries share a drain pass (the #538 discriminator needs the sibling in the same pass):
+        // two separate Tells race the off-mailbox append loop, which may append and drain the first alone.
+        rec.Tell(new VirtualTagActor.DependencyValueBatch(new[]
+        {
+            new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow),
+            new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow),
+        }));
 
         await AwaitAssertAsync(async () =>
         {
@@ -696,8 +716,13 @@ public sealed class ContinuousHistorizationRecorderTests : TestKit
             drainInterval: TimeSpan.FromMilliseconds(20),
             maxWriteAttempts: 2));
 
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow));
-        rec.Tell(new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow));
+        // ONE burst, so both entries share a drain pass (the #538 discriminator needs the sibling in the same pass):
+        // two separate Tells race the off-mailbox append loop, which may append and drain the first alone.
+        rec.Tell(new VirtualTagActor.DependencyValueBatch(new[]
+        {
+            new VirtualTagActor.DependencyValueChanged("Good.Tag", 1.0, DateTime.UtcNow),
+            new VirtualTagActor.DependencyValueChanged("Bad.Tag", 2.0, DateTime.UtcNow),
+        }));
 
         await AwaitAssertAsync(async () =>
         {
@@ -1721,9 +1746,14 @@ public sealed class ContinuousHistorizationRecorderTests : TestKit
                 return AppendBatchYieldingAsync(entries);
             }
 
-            foreach (HistorizationOutboxEntry entry in entries)
+            // One lock over the whole batch (Monitor is re-entrant; AppendCore takes it too) so a concurrent drain peek
+            // sees the batch all-or-nothing, as it does a production group commit.
+            lock (_gate)
             {
-                AppendCore(entry);
+                foreach (HistorizationOutboxEntry entry in entries)
+                {
+                    AppendCore(entry);
+                }
             }
 
             return ValueTask.CompletedTask;