Structures don't spawn their respective mobs inside of structure boundaries in certain buffet type worlds. They are also able to spawn mobs that are not normally supposed to spawn within their boundaries.
Some examples of this are:
- Witches not spawning in witch huts and the cats spawned from generating these witch huts can also be non-black cats.
- Guardians not spawning in ocean monuments.
- Pillagers not spawning in pillager outposts.