Information

Minecraft version: 1.21.1
Modloader: Forge
Environment: Singleplayer / Multiplayer

Mod name: Biome Spawn Point
Mod version: 1.2.3

Question
I do this in spawnbiomes.txt :
undergarden:dense_forest,
undergarden:frosty_smogstem_forest,
undergarden:smogstem_forest,
undergarden:wigglewood_forest
But I have the impression that the mod doesn't spawn in modded or custom dimensions. If so, how does it work? Thanks a lot!