There's a glitch going on the generation of "Mushroom Islands"

The biome (Mushroom_Field and Mushroom_Field_Shore) are known for the fact that no mobs can spawn on it but sometimes when those biome are generated in the game near the shore another biome (mostly ocean) can override the mushroom's one and put patches of others biomes on the mushroom ones therefore creating some spawn area for mobs on a place where no mob should spawn. if you look at the screenshot i added to this ticket you can see the "patches" of grass_block being created on the mushroom_field/_shore allowing mob to spawn on it.