Biomes with multiple words like soul sand valley don't work. I tried soul_sand_valley, SoulSandValley and "soul sand valley" formats and none seem to work. Only biomes with one word work like plains or swamp.

Turns out I was grabbing the biome category not the actual name, this has been fixed for version V2.1.4, which should be releasing tonight, soul_sand_valley, Soul_Sand_Valley, & minecraft:soul_sand_valley are all correct and now working examples, "soul sand valley" will not be working for the time being

badlands doesn't seem to work either

mushroom_fields don't work but just mushroom work