diff --git a/src/core/thread/mle.cpp b/src/core/thread/mle.cpp
index f2f4a39a2..49eedd2d4 100644
--- a/src/core/thread/mle.cpp
+++ b/src/core/thread/mle.cpp
@@ -3598,6 +3598,7 @@ void Mle::HandleChildUpdateRequest(const Message &aMessage, const Ip6::MessageIn
         tlvs[numTlvs++] = Tlv::kLinkFrameCounter;
         break;
     case kErrorNotFound:
+        challenge.mLength = 0;
         break;
     default:
         ExitNow(error = kErrorParse);
@@ -3655,7 +3656,7 @@ void Mle::HandleChildUpdateRequest(const Message &aMessage, const Ip6::MessageIn
     }
 
 #if OPENTHREAD_CONFIG_MULTI_RADIO
-    if (aNeighbor != nullptr)
+    if ((aNeighbor != nullptr) && (challenge.mLength != 0))
     {
         aNeighbor->ClearLastRxFragmentTag();
     }
diff --git a/src/core/thread/mle_router.cpp b/src/core/thread/mle_router.cpp
index 7e2851016..78667457b 100644
--- a/src/core/thread/mle_router.cpp
+++ b/src/core/thread/mle_router.cpp
@@ -2507,7 +2507,14 @@ void MleRouter::HandleChildUpdateRequest(const Message &         aMessage,
     }
 
 #if OPENTHREAD_CONFIG_MULTI_RADIO
-    child->ClearLastRxFragmentTag();
+    // We clear the fragment tag only if the "Child Update Request" is
+    // from a detached child trying to restore its link with its
+    // parent which is indicated by the presence of Challenge TLV in
+    // the message.
+    if (challenge.mLength != 0)
+    {
+        child->ClearLastRxFragmentTag();
+    }
 #endif
 
     SendChildUpdateResponse(child, aMessageInfo, tlvs, tlvslength, challenge);