When Steam Play's Proton version is set to "Proton 5.13-2", games that use it will not launch (basically all non-native Linux games) due to an error with "pressure-vessel".
Possibly related - https://github.com/NixOS/nixpkgs/issues/100655
This impacts Steam from the repos and installed as a Flatpak. Toggling LSI has no impact. The only thing the makes non-native games function is forcing Proton 5.0-10 which reduces performance and even breaks some games.
When Steam's Proton version is set to "5.13-2", the following errors spam the dmesg log:
[ 9747.296557] traps: pressure-vessel trap invalid opcode ip:7fbc228bed89 sp:7ffce2834140 error:0 in libc-2.31.so[7fbc228bd000+175000]
When launching any game with Proton 5.13-2, the following error is shown in Steam's stdout:
pressure-vessel-launch: Can't connect to peer socket: Could not connect: No such file or directory