Recently I tried putting equipment on a mob with the NBT tag the one that lets them equip items or hold out different types of items such as swords for instance, however after awhile the items randomly disappear from the equipped mob.

What I thought was going to happen is that the equipped items would stay on the mob and the mob I tested was a zombie pigman, but they eventually vanished from said zombie pigman even if the items were at full durablity.

To reproduce the bug:

Try giving a mob with the NBT tag an item they can equip.
Do something else for awhile
Come back and find out the item they were wearing is now gone. I did not even leave the chunk and this occurred.

I've found out that it mainly effects the helmet slot, but not the other slots that a mob can equip.