When mobs fall from a high place no experience is given, I have an mob farm that works that way and it wont work I've done it on servers and on my own world. sand baby zombies take no fall damage. Its been happening since 1.13.
MCPE-75731
Mobs dont give exp when dying of fall damage
Resolved
Duplicate
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This is working as intended. Mobs only drop experience when killed by the player.