We have a server that is years old, and once every few months we have to do maintenance on it because sometimes a random mob will die, or for one reason or another will produce an item. Then, a zombie picks up that item, causing it to become permanent. So, to combat this, we have to do a /kill on each of the hostile mob types, followed by a /kill @e[type=item]. This only gets the loaded areas, so it's still difficult.

Just because a skeleton stepped in some lava flow shouldn't be the reason a zombie now is carrying a bone in its hand and is forever wandering the server now. I'd propose a fix of tracking where an item originates, and only use items from a player, dropper, or dispenser to make a mob persistent when picking up items.

Expected behavior: Mobs should despawn when left alone and not interacted with deliberately by a player.

Current behavior: Zombies (and other mobs) can become persistent on their own by picking up random items dropped by mobs that happen to die on their own, preventing them from ever despawning in the first place.