The official Minecraft Wiki states that no hostile mobs spawn in the Mushroom Fields and Mushroom Fields Shore biomes. The phantoms break this game mechanic by spawning in these biomes anyway.

No mobs other than mooshrooms spawn naturally in this biome, including the usual night-time hostile mobs; this is also the only biome where mooshrooms can spawn. This also applies to caves, abandoned mineshafts, and other dark structures, meaning exploring underground is mostly safe.

Reference:

• https://minecraft.gamepedia.com/Biome
• https://minecraft.gamepedia.com/Mushroom_Fields
• https://minecraft.gamepedia.com/Mushroom_Fields#Mushroom_Field_Shore