Check by setting blocks with BlockBagExtend for changed materials.

worldedit-core/src/main/java/com/sk89q/worldedit/extent/inventory/BlockBagExtent.java

@@ -86,6 +86,7 @@ public class BlockBagExtent extends AbstractDelegateExtent {
         if (blockBag != null) {
             BlockState existing = getExtent().getBlock(position);
 
+            if (!block.getBlockType().equals(existing.getBlockType())) {
                 if (!block.getBlockType().getMaterial().isAir()) {
                     try {
                         blockBag.fetchPlacedBlock(block.toImmutableState());
@@ -108,6 +109,7 @@ public class BlockBagExtent extends AbstractDelegateExtent {
                     }
                 }
             }
+        }
 
         return super.setBlock(position, block);
     }