On a recently-downloaded-off-realms survival world, I had a bunch of chickens in a farm that had later for some reason all escaped at once. As I was trying to get rid of them all with my enchanted netherite axe (being specific to help ya'll track down the bug), I realized that none of them were dropping xp, even though they were still dropping raw chicken and feathers as intended. I tried turning mob drops off and on and even exited and re-entered the world, but they still didn't drop xp, although other mobs dropped xp as normal.