In bastion remnants, natural mob spawns are based biome. This means that the mobs which generated in the structure, (Piglins, Piglin Brutes, magma slimes, and hoglins) will not continue to spawn within the structure, specifically if they are in warped forests which only spawn enderman.
Seeing enderman and ghasts in bastion remnants doesn't really fit to the structure. This also means that zombie pigmen may spawn inside.
In witch huts, the mob that generates there also spawns there. For nether fortresses, ocean monuments and pillager outposts, the structure spawns specific mob types relating to the structure. For igloos, aquatic structures and Zombie villages, the mobs that generate there can spawn there, based on biome.
Woodland mansions similarly to bastion remnants, don't spawn the mobs that are there. However, the mobs that generate in Woodland mansions do not spawn naturally.
In nether fortresses, blaze spawners exist and blazes spawn there. However, in bastion remnants, magma cubes may not, depending on biome.
The solutions I see are:
A. Making it so that bastion remnants don't spawn in biomes where piglins don't spawn.
B. Making it so that only mobs relating to bastion remnants spawn in bastion remnant bounding box.