If I run the following script:

swamp = forBiomes("minecraft:swampland")
swamp.removeDicTypes("WET")

wet = forBiomesOfTypes("WET")
wet.removeSpawn("net.minecraft.entity.passive.EntityCow", "CREATURE")

I would expect to see cows still spawning in swamps. What actually happens is that cows get removed from Swampland as if the biome were still WET.

Presumably the forBiomesOfTypes() is being run against the original biome dictionary, instead of the one modified by removeDicTypes(). It's possible that this is intended behaviour, but it doesn't seem correct to me.

I can't see a workaround short of explicitly removing "minecraft:swampland" from wet, which kinda misses the point of the first part.