I added some biomes from Terralith to the spawning parameters in the config file but for some reason they still don't spawn.

I think that has to do with Terralith, it happened for me with Mythic Mounts. Some spawned, some didn't.

Which version for which Minecraft? Paste also your current configuration

Oh yeah I forgot! It's version I.7.4 for 1.18. I added the biomes using the Mod Menu, making sure to close and reload the game. I also added the Mooblooms to Meadows since I felt they fit there too and I did find them in Meadows so I know they are at least being added in vanilla biomes.

Oh it's happening with Mythic Mounts too? I'm sure it's something to do with Terralith too but it seems other mods have been able to make it work.

Having this issue myself as well, think it's because of the way Terralith is handling biome tags?

Earth2Java-1.8.0+1.18.2.jar
Terralith_v2.2.1a.jar
fabric-api.0.48.0.jar

Can drop more info if needed.