At first I thought my issue was the same as #27 but even without Biome Makeover I'm not getting any Cluckshrooms. I tried adjusting the spawn rate incrementally higher several times eventually reaching 999 and all it did was make Mooshrooms spawn less. Tbh it only seems to be affecting the Mushroom biome because I can make them spawn in Dark Forests fine.

PS : I think it might be something to do with the API. Maybe updating to the latest Fabric might fix it? Because more recent mods aren't having issues with mob spawns in the Mushroom biome.