When downloading a realm which contains custom dimensions via a datapack, it successfully saves the regions for the dimension. However, when uploading the world to the realm again, it doesn't keep those region files. Essentially, this causes the dimensions to be reset upon reuploading the world

From what I can tell, Realms deletes/doesn't load files in the world it thinks it "Doesn't need" (Back in 1.16.1, it wouldn't upload custom dimension datapacks at all, and to my knowledge it still doesn't work with dimensions which have custom carvers/biomes) and the system hasn't been updated to accommodate for the new dimension code (And while that can be expected for "Experimental features", the fact that it still saves the data correctly, and that the problem can be fixed with a relatively simple change in the way realms loads data, I think it should be fixed in this instance, for parity if nothing else)