Summary:
The timer, how long ago a mob was attacked by a player is lost upon reloading the world, preventing them from dropping xp. Mobs drop xp, if it was attacked by a player 5 seconds ago or less. This can be reproduced by knocking any mob that drops experience off a cliff (make sure, that it doesn’t take more than 5 seconds to fall), reload the world during the fall and you will notice, that it doesn’t drop experience.

Steps to reproduce:

1. Knock the mob dropping xp off the cliff so that it dies (make sure that the fall time does not exceed 5 seconds).
2. Reload the world during a mob fall.

Observed results:
After the fall, the mob does not drop xp.

Expected results:
Mob drops xp after falling.

Video:
Minecraft 1.19 - Singleplayer 2022-06-18 10-56-15.mp4