item.h:

  Fix for fix for bug#9728 decreased functionality in "on duplicate key update"
  Have to return false to set flag for whole expression.

sql/item.h

@@ -336,7 +336,7 @@ class Item {
   virtual bool set_flags_processor(byte *args)
   {
     this->item_flags|= *((uint8*)args);
-    return true;
+    return false;
   }
 };