fix stackable sboxes

src/main/java/carpet/mixins/ItemEntityMixin.java

@@ -82,6 +82,7 @@ public abstract class ItemEntityMixin extends Entity implements ItemEntityInterf
         ItemStack otherStack = other.getStack();
         if (selfStack.getItem() == otherStack.getItem()
                 && !InventoryHelper.shulkerBoxHasItems(selfStack)
+                && !InventoryHelper.shulkerBoxHasItems(otherStack)
                 && selfStack.hasTag() == otherStack.hasTag()
                 && selfStack.getCount() + otherStack.getCount() <= SHULKERBOX_MAX_STACK_AMOUNT)
         {