For many mobs, drops will have a chance to spawn and are not a guaranteed outcome. Does this affect certain mobs specifically? Which mobs have you tested with?
It also goes the same for me. I made a mob farm out in the ocean and the skeletons dropped their armor when they shouldn't (it must be a player kill for the armor to be dropped, I suppose).
For many mobs, drops will have a chance to spawn and are not a guaranteed outcome. Does this affect certain mobs specifically? Which mobs have you tested with?