diff --git a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs
index 4730777a..bd177379 100644
--- a/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs
+++ b/tests/Drivers/ZB.MOM.WW.OtOpcUa.Driver.AbLegacy.Tests/FakeAbLegacyTag.cs
@@ -115,8 +115,13 @@ internal class FakeAbLegacyTag : IAbLegacyTagRuntime
 /// <summary>Test factory for creating fake AbLegacy tags.</summary>
 internal sealed class FakeAbLegacyTagFactory : IAbLegacyTagFactory
 {
-    /// <summary>Gets the collection of created tags, keyed by tag name.</summary>
-    public Dictionary<string, FakeAbLegacyTag> Tags { get; } = new(StringComparer.OrdinalIgnoreCase);
+    /// <summary>
+    ///     Gets the collection of created tags, keyed by tag name. Concurrent on purpose (#573): the driver's
+    ///     connectivity probe is on by default and creates its <c>S:0</c> runtime from a thread-pool thread
+    ///     while a test thread creates the tag under test, so a plain <see cref="Dictionary{TKey,TValue}"/>
+    ///     here lost inserts (the "given key was not present" flake, ~1 run in 15).
+    /// </summary>
+    public System.Collections.Concurrent.ConcurrentDictionary<string, FakeAbLegacyTag> Tags { get; } = new(StringComparer.OrdinalIgnoreCase);
 
     /// <summary>Gets or sets an optional customization function for tag creation.</summary>
     public Func<AbLegacyTagCreateParams, FakeAbLegacyTag>? Customise { get; set; }