While trying out a mod called Plants vs Zombies made with Mcreator, I discovered that custom hostile mobs will spawn in Mushroom biomes.
I then created a test entity in my own mod, and I found out that if the mob is set to spawn like a monster, it will still spawn in Mushroom biomes by default. There is the option to choose which biome the hostile mob can spawn in, but that would require selecting every biome except Mushroom biomes, which would take a while.
To solve this. the best way would be to either exclude Mushroom biomes from monster spawning by default, or to add a biome blacklist.
The entity element in my workspace is named TestEntity.