Background: My world, Alendyias, is more than a little interesting. As the Fracture in Reality grows and expands, it takes apart and reforms worlds inside itself, and over the ages, this has formed one big world: Alendyias. Living things are often mutated at least a little by this trip through chaos itself, creating things like goblins and giant spiders, and these are called monsters.
When these monsters are slain, though, there is a burst of light and chaotic (magical) energy, and when that clears, what remains are the monster's Drops: objects formed or altered by the combination of the monster's fading life force and the magical energy inside them.
The former are usually body parts; pelts, claws, horns, and the like. However, the latter can be any junk inside or on the monster; a good example would be the personal belongings (clothes, money, jewelry, perhaps even weaponry) of a Catfish Monster's victims. Somehow, upon its death, its accumulated life force and magical energy not only recreates but enchants these items, leaving behind valuable loot.
Good so far, yes? However, what if a monster ate something before it died? An intact body, or even a skeleton, inside a monster could quite feasibly be turned into an undead monstrosity upon their consumer's death. So my question is: How Can Dead Monsters Not Spawn Undead?
EDIT: Monsters are not in the habit of swallowing prey whole and not digesting it; rather, spawning undead happens when a monster is killed before it can really digest swallowed prey, and its lifeforce subsequently reanimates its victims. Also, digesting bone is relatively unusual and difficult to do; so skeletons should not be unheard of without some sort of natural undead prevention method-which is exactly what my question is asking for.
As always, I appreciate input and feedback, and if you decide you need to down- pr close-vote, please give me an explanation so I can do better in the future. Thank you all!