The bug

Fish of all types (cod, salmon, pufferfish, tropical) occasionally drop bones when killed.

In Java Edition, fish do not drop bones (aquatic parity). Considering no other mob other than skeletons drop bones, and fish clearly do not have large femur bones which the bone texture represents, Java's behavior is far more likely to be the correct one. If Java fish begin dropping bones, I will close this ticket.

How to reproduce

Spawn and kill some fish. It appears to be a fairly uncommon drop, but you'll be sure to encounter it fairly soon. See the attached screenshot.