They're robbing it. To aviod that I put extracted supers directly on hives and they clean it up without thinking they are robbing it becuase it's now part of their hive. The times I leave them out to get "cleaned" they invariably rob it such that they damage any newly drawn comb. If the comb is older (thicker) and may have had some brood in it at some point they don't seem able to damage it at all.