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 a major issue but is outside the scope of HotM. The fix for this is being moved to its own mod.

I have released the Dimension Update Fixer mod which fixes this issue.