Zombie sieges can happen on mushroom islands

The bug

Zombie sieges can spawn in "Mushroom fields" biomes, even though normally, no mobs can spawn in that biome (except phantoms).

To reproduce

1. Create a new buffet world with "Mushroom fields" as biome
2. Place 20 beds and spawn in 20 villagers
3. Place a bell near the villagers
4. Run /time set 17999 repeatedly
   → Zombies spawn

Original description

having zombie sieges on a mooshroom island(mooshroom field biome), not sure if it is a bug or intended though. seed:101

are not mooshroom biomes supposes to be hostile mob free?