A composter will delete bonemeal under certain circumstances if a hopper below cannot remove the bonemeal item. To reproduce, place a composter on top of a hopper and fill each slot of hopper with a single item that is not bonemeal. Fill the composter to level 7 and observe as the composter is emptied but bonemeal does not get sucked into hopper nor does it pop out like it does when there is no hopper under the composter.