I have a realm in which I would like to use a custom dimension for creative purposes. I made some builds in another world and transferred it over to the new dimension. Opening the world in singleplayer and going to the custom dimension shows that this transfer was successful (I copied the region and other data from the first world into the folder for the custom dimension). However, upon uploading the world to my realm, any changes in the overworld are preserved, yet the custom dimensions are completely reset.

In the realms-upload.log file (which is attached) from the realm backup, it reads:

Removed world/dimensions/minecraft/custom_dimension/region/r.0.0.mca: not used

It reads this for all the regions of the custom dimension I'm trying to upload.