Total random guess but maybe some system component are run by the user and the other user has issues because it is not its own session?
For pipewire I have created a config file:
$ cat .pulse/client.conf
default-server = 127.0.0.1
But I think issues like this would also appear with native Linux games.
Did you try a real session from the problematic user instead of using
su?
This works fine. (after removing default-server from .pulse/client.conf)
A friend of mine uses a similar setup with sway. He uses this command cd /tmp && sudo -u gaming PULSE_SERVER=tcp:localhost:4713 STEAM_RUNTIME=0 steam from the main user and it works without any issue.