diff --git a/src/Nethermind/Nethermind.Blockchain.Test/BlockTreeTests.cs b/src/Nethermind/Nethermind.Blockchain.Test/BlockTreeTests.cs
index 8d0d91469757..b4dfcc434da1 100644
--- a/src/Nethermind/Nethermind.Blockchain.Test/BlockTreeTests.cs
+++ b/src/Nethermind/Nethermind.Blockchain.Test/BlockTreeTests.cs
@@ -2626,6 +2626,45 @@ public void UpdateMainChain_WhenBeaconSyncAndFcuCycleRepeatedTwice_ClearsStaleMa
         blockTree.IsMainChain(sibling1.Header).Should().BeFalse("sibling1 must be orphaned");
     }
 
+    [Test, MaxTime(Timeout.MaxTestTime)]
+    public void UpdateMainChain_WhenForwardProcessingWithBeaconSyncedDescendants_DoesNotClearMarkers()
+    {
+        (BlockTree blockTree, Block genesis) = BuildBlockTreeWithGenesis(forceUpdateHead: true);
+
+        Block[] chain = BuildAndSuggestChain(blockTree, genesis, 4);
+        blockTree.UpdateMainChain(new[] { chain[0] }, wereProcessed: true, forceUpdateHeadBlock: true);
+        for (int i = 1; i < chain.Length; i++)
+            blockTree.UpdateMainChain(new[] { chain[i] }, wereProcessed: false);
+
+        // Forward processing H=2 (forceUpdateHeadBlock: false) must not clear H=3, H=4
+        blockTree.UpdateMainChain(new[] { chain[1] }, wereProcessed: true, forceUpdateHeadBlock: false);
+
+        blockTree.IsMainChain(chain[2].Header).Should().BeTrue("H=3 marker must survive");
+        blockTree.IsMainChain(chain[3].Header).Should().BeTrue("H=4 marker must survive");
+    }
+
+    [Test, MaxTime(Timeout.MaxTestTime)]
+    public void UpdateMainChain_WhenFcuForwardReorgToLongerChain_ClearsStaleMarkersAboveNewHead()
+    {
+        (BlockTree blockTree, Block genesis) = BuildBlockTreeWithGenesis(forceUpdateHead: true);
+
+        Block[] chainA = BuildAndSuggestChain(blockTree, genesis, 4);
+        blockTree.UpdateMainChain(new[] { chainA[0] }, wereProcessed: true, forceUpdateHeadBlock: true);
+        for (int i = 1; i < chainA.Length; i++)
+            blockTree.UpdateMainChain(new[] { chainA[i] }, wereProcessed: false);
+
+        // FCU to chain B at H=3 (forceUpdateHeadBlock: true) must clear A4
+        Block b1 = Build.A.Block.WithNumber(1).WithParent(genesis).WithExtraData([0xBB]).TestObject;
+        Block b2 = Build.A.Block.WithNumber(2).WithParent(b1).WithExtraData([0xBB]).TestObject;
+        Block b3 = Build.A.Block.WithNumber(3).WithParent(b2).WithExtraData([0xBB]).TestObject;
+        blockTree.SuggestBlock(b1);
+        blockTree.SuggestBlock(b2);
+        blockTree.SuggestBlock(b3);
+        blockTree.UpdateMainChain(new[] { b1, b2, b3 }, wereProcessed: true, forceUpdateHeadBlock: true);
+
+        blockTree.IsMainChain(chainA[3].Header).Should().BeFalse("A4 stale marker must be cleared");
+    }
+
     [Test, MaxTime(Timeout.MaxTestTime)]
     public void FindBlock_WhenBlockOrphanedAfterReorgInPoS_ReturnsNull()
     {
diff --git a/src/Nethermind/Nethermind.Blockchain/BlockTree.cs b/src/Nethermind/Nethermind.Blockchain/BlockTree.cs
index 42c493d714a0..37913f3a8308 100644
--- a/src/Nethermind/Nethermind.Blockchain/BlockTree.cs
+++ b/src/Nethermind/Nethermind.Blockchain/BlockTree.cs
@@ -1025,8 +1025,11 @@ public void UpdateMainChain(IReadOnlyList<Block> blocks, bool wereProcessed, boo
             }
 
             // Clear stale canonical markers above the new head left by beacon sync.
-            // Covers both same-height FCU (previousHeadNumber <= lastNumber) and ePBS FCU to ancestor.
-            ClearStaleMarkersAbove(Math.Max(previousHeadNumber, lastNumber), batch);
+            // Only needed on FCU reorgs (forceUpdateHeadBlock == true). During forward sync
+            // (BlockDownloader) and forward processing (BlockchainProcessor), the markers above
+            // are either not yet set or belong to the same chain and must not be cleared.
+            if (forceUpdateHeadBlock)
+                ClearStaleMarkersAbove(Math.Max(previousHeadNumber, lastNumber), batch);
 
             for (int i = 0; i < blocks.Count; i++)
             {