So i was joining a realm and then i canceled it and went to join a server and for some reason after i joined the server it connected me to my realm as well as the server the image i've added is what happened to the end dimension which is what i was in in my realm but the server is in the overworld and the chat is from the server as well and the Tab screen is the server everything else is from my realm i've noticed that if i cancel far into joining the realm it will still join even if your on the main menu of the game

How to replicate: join a realm and wait a few seconds depending on how long it takes to connect and then cancel it after it cancels it should still connect you to the realm and this causes problems with other parts of minecraft.

Tested multiple times in 1.16.4 i haven't tested it 1.16.5-rc1 but there's no shown fixes for this so i assume it is the same there.

This seams to be based off of timing so try a few times

In the attachments there's an image and the endstone you see is what is surpose the be the ground but is now a wall on my screen from my realm which is not a part of the server