There appears to be an incompatibility with Realms servers and JSON based custom worlds introduced in 1.16.
Now this incompatibility could very easily be tied simply to JSON based worlds with more than the standard three dimensions. I am currently working on an adventure style world for my friends to play on using Realms. For our purposes I have created a JSON based custom world with five additional dimensions beyond the original three.
The JSON based world works like a dream in single player, and on LAN. Sadly not all the players for this adventure can connect via LAN.
Attached below is the JSON used for the custom world, as well as a few images about the process.
The world itself uploads with no issues, but when attempting to connect the the world the loading screen never ends. It is not possible to proceed beyond that point. I understand that the JSON based custom worlds are experimental, but I also feel like this incompatibility is something that should be brought to your attention.