If you make any hostile mob fall to the ground and you are over 128 blocks above the ground, the mob will despawn before dying for hitting the ground.
Steps to reproduce:
1.Make hostile mobs spawn and fall at more than 128 block from the ground.
2.Watch if there are drops on the ground, if there aren't, the bug has probably worked. Or invite another player in gamemode 3 to see if the mob hits the ground or not.