If you join a realm server then quickly join a singleplayer server after pressing cancel it still loads the realm while keeping the location, but it also loads the super flat putting you at y:4 so you can see cave systems and other game-breaking things.
Environment
Windows 10
Java: 1.16.1
Specs: GTX 1080Ti Graphics Card
i7-7800x 3.50 GHz CPU
31.69 GB RAM
1920x1080 144 hz monitor
Hi there!
This looks like a duplicate of MC-74984.