diff --git a/engines/ARIEC61850.lock.json b/engines/ARIEC61850.lock.json
index 095525e0e..9d631dc08 100644
--- a/engines/ARIEC61850.lock.json
+++ b/engines/ARIEC61850.lock.json
@@ -2,7 +2,7 @@
   "schemaVersion": 1,
   "repository": "masarray/ARIEC61850",
   "ref": "main",
-  "commit": "902fc828dcf976a705dc73b88f1e9c42e5bf6a93",
-  "sourcePullRequest": 54,
+  "commit": "8b46cfed0614a4d0b0a5bf73fa7a6e77cf6fe817",
+  "sourcePullRequest": 55,
   "purpose": "Immutable ARIEC61850 revision for ARSAS CI, tests, packaging, diagnostics, and release provenance."
 }
diff --git a/tests/ARSAS.Tests/Iec61850EngineTimestampIntegrationTests.cs b/tests/ARSAS.Tests/Iec61850EngineTimestampIntegrationTests.cs
new file mode 100644
index 000000000..08b624471
--- /dev/null
+++ b/tests/ARSAS.Tests/Iec61850EngineTimestampIntegrationTests.cs
@@ -0,0 +1,44 @@
+using System.Buffers.Binary;
+using System.Globalization;
+using AR.Iec61850.Binding;
+using AR.Iec61850.Mms;
+using ArIED61850Tester;
+
+namespace ARSAS.Tests;
+
+public sealed class Iec61850EngineTimestampIntegrationTests
+{
+    [Fact]
+    public void PinnedEngine_Preserves_2006000_Fraction_And_Arsas_Rounds_It_To_201ms()
+    {
+        var seconds = new DateTimeOffset(2026, 8, 13, 10, 0, 31, TimeSpan.Zero).ToUnixTimeSeconds();
+        Span<byte> bytes = stackalloc byte[8];
+        BinaryPrimitives.WriteUInt32BigEndian(bytes[..4], checked((uint)seconds));
+
+        // IEC 61850 fractional-second bytes from the engine regression case:
+        // 0x335A86 / 2^24 = 0.200600028... s -> 31.2006000 at .NET tick resolution.
+        bytes[4] = 0x33;
+        bytes[5] = 0x5A;
+        bytes[6] = 0x86;
+        bytes[7] = 0x00;
+
+        var utc = Iec61850UtcTime.FromBytes(bytes);
+        var decoded = Iec61850TimestampDecoder.Decode(MmsDataValue.UtcTime(utc));
+
+        Assert.True(decoded.IsDecoded);
+        Assert.True(decoded.DisplayTime.EndsWith("31.2006000", StringComparison.Ordinal), decoded.DisplayTime);
+
+        Assert.True(
+            DateTime.TryParseExact(
+                decoded.DisplayTime,
+                "yyyy-MM-dd HH:mm:ss.fffffff",
+                CultureInfo.InvariantCulture,
+                DateTimeStyles.None,
+                out var parsed),
+            decoded.DisplayTime);
+
+        Assert.Equal(
+            "31.201",
+            Iec61850TimestampPresentation.FormatMilliseconds(parsed, "ss.fff"));
+    }
+}