Skeletons don't drop bones

The version is actually 1.6.0.14, but above drop list doesn't contain.

I built a skeleton trap from a spawner. It worked fine for a long time.

All of a sudden the skeletons don't drop bones anymore. I tried killing them with and without looting 3 sword. I tried closing and reopening world. I tried removing all packs from world. I tried closing and reopening app. I even tried rebooting phone.

They no longer drop bones for me. Survival world, hard difficulty, no cheats.