Hi, it seems that using a custom surface builder will cause to prevent showing the new custom biome on the biome list of a single-biome world type/preset.  The custom sufrace builder that I'm using is called "volcanic.json" (see all .json on attached files), and the customized biome that is using this is called "birch_forest.json". Also I will provide some pictures of the .json locations (maybe it could be wrong located and causing this issue?).

Thanks for reading.