https://www.curseforge.com/minecraft/mc-mods/mob-population-control

Mob Population prevents mobs that pick up junk items on the configuration list from being flagged as permanent. Instead, the mobs will despawn normally as players leave the area.

This fix has already been implemented by UT (in some way):

Mob Despawn Improvement: Mobs carrying picked up items will drop their equipment and despawn properly.

Consider adding mod this to the list of obsolete mods.