Your system information

• Steam client version (build number or date): February 11 2020 19h27
• Distribution (e.g. Ubuntu): Debian Buster
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: No

My system link: https://gist.github.com/kadogo/a72b612e48ede28a5bd7495b268de99d

Please describe your issue in as much detail as possible:

I filled my non steam game option like in my screenshot

I add printf "%s %s\n" "$here/pressure-vessel/bin/pressure-vessel-unruntime" "$@" in /home/user/.steam/steam/steamapps/common/SteamLinuxRuntime/run-in-scout before the exec for seeing the command that launch the game

Steps for reproducing this issue:

1. I launch my non steam game and I see the next result in my terminal

/home/user/.steam/steam/steamapps/common/Dead Cells/start.sh DRI_PRIME=1
%command%

1. I did the same test with another game (Steam game this time)

/home/user/.steam/steam/steamapps/common/SteamLinuxRuntime/pressure-vessel/bin/pressure-vessel-unruntime --
/home/user/.steam/steam/steamapps/common/CrossCode/CrossCode --in-process-gpu
--startedFromSteam 

I think that non steam games interpret the launch options like something to be added after the normal command.

If I can help with other test let me know.

Cheers.