I inserted a single bone into a millstone via right-click, hit the hand crank five times, and nothing came out. If I right click the millstone now, no bone pops back out; but I can put other items in and take them back out. The bone seems to have vaporized.
This was my very first (and so far, only) time using the new GUI-less millstones, and in fact, my only time using a millstone post-AAAAAAAAAAHHHH, period. I don't have much food to experiment with it in survival, and failed to reproduce it in creative. I'm hoping that others may have similar reports, or alternative explanations that I did not yet consider.
Pictures of mill stone:
The area looks exactly as it did when I did the grinding. The visible campfire and torch were already burnt out. The mill stone and hand crank are both 3+ blocks from a boundary. I searched the water thoroughly for items, of course.
BTW V4.ABCEEEFAb