I upgraded from Unity 6.0.24f1 to 6.0.31f1, and now my cameras are no longer rendering in game mode.
The camera does however still render when I build the game.
The below screenshots feature a brand new scene, untouched—so this appears to be endemic to this version of unity, or somehow caused during the upgrade process.
I can only offer the usual tryfix, to be applied every time you run into something out of the ordinary and particularly when changing editor versions: delete the Library folder.
Other than that make sure you have the latest (stable, preferably Studio/Pro) graphics driver installed.
Sadly, deleting the Library folder didn’t work.
This is running on a MacOS system, so the graphic drivers are out of my hand!
The camera does render the interface, provided it’s in Screen Space Overlay, which leads me to suspect the bug is that the camera is not rendering the world
Based on this thread, which reported something similar, deleting the default project settings folder appears to have worked!