Encountered this issue with my mod immortius/chunkbychunk#10 . While chunks from custom dimensions (added programmatically) seem to save correctly, the existence of the custom dimensions does not serialize/deserialize with fabric in 1.19 (specifically the LevelStems/Levels). They do serialize/deserialize correctly with forge. I've worked around this by recreating them programmatically when loading, but this wasn't necessary in 1.18/1.17.

This is a deviation from previous behaviour, but I don't know if it is intended or a side effect of changes to minecraft rather than fabric... or potentially I've been setting up dimensions wrong the whole time 🤷

@immortius Fabric intentionally strips the dimension data from level.dat because people have reported this causes issues when removing a dimension mod. According to them it regenerates on launch - however I am unsure if they are correct.

If you rely on custom dimension's WorldGenSettings being saved in level.dat file, that would cause an issue.

My mechanism for adding the dimensions is probably non-standard - since it isn't using datapacks. So if the removal of dimension data is intentional then I'ld say it is working as intended.