This is caused by TypeReferences.CHUNK_GENERATOR_SETTINGS being registered without HotM's Nectere ChunkGenerator ("hotm:nectere"). This means that when various datafixers attempt to fix missing-dimension data from a previous versions's save file, they fail to load the dimension data, meaning that only forcefully loaded dimensions remain ("minecraft:overworld" and "hotm:nectere").
This is caused by
TypeReferences.CHUNK_GENERATOR_SETTINGS
being registered without HotM's Nectere ChunkGenerator ("hotm:nectere"). This means that when various datafixers attempt to fix missing-dimension data from a previous versions's save file, they fail to load the dimension data, meaning that only forcefully loaded dimensions remain ("minecraft:overworld" and "hotm:nectere").