I've built my base in a mushroom fields biome specifically to avoid hostile mobs, but phantoms continue spawning when I'm in the biome. The Minecraft Wiki says "If the player hasn't slept in three nights, Phantoms spawn as soon as the player is no longer within the biome at night," so that makes me think this might not be working as intended!

https://minecraft.fandom.com/wiki/Mushroom_Fields