a goblin whelp holding a 1 handed hammer not in inventory
 

I have been hunting "a goblin whelp" in Butcherblock Mountians. 4 times I have killed one that had a one handed hammer graphic, but it is not in their inventory when I loot it.

I cannot reproduce. They do not have a ornamental item currently assigned.

Mobs will display a weapon given to them by a player, but if the mob wasn’t charmed at the time the weapon was given then it won’t drop the weapon (unless it is magic and got disarmed).

Another way this bug may have happened is mobs continue to display held items even after they are pickpocketed (tbh I thought this only applied to non weapons like shields).