I do not want to jinx it and will test it further but it did not logout anymore since I have used this. It was the answer to someone else having the same issue years ago in another distribution.
Edit: This seems to be just a temporary solution that is disabled again after a restart of the system.
Also I have changed graphics settings, to no avail. When the above command is not set it logs out of Manjaro sooner or later while playing.
It is odd though because one would expect to get a message that the system would like to log out, because it does it without that setting. But when it is set then I can play without interuption/logout and no message that it would like to.
Sounds very much like automatic screen locking but I don’t know why the behaviour would change after an update.
If you’re playing with a controller only then KDE doesn’t account for game controller events in the idle timer. So unless the game itself disables screenlock/screensave/powersave then this is exactly what happens.
You can manually disable it by middle-clicking the Power Management applet (which is probably hidden on a desktop PC). Or install the gamemode package and use that. In Steam;