Solved!
Paradoxically, by adding the PROTON_LOG=1 %command%, Steam manages to launch the game instead of spouting EINVAL: invalid argument. Tested multiple times, the cmdline is the same except for the var def:
/bin/sh\0-c\0PROTON_LOG=1 /home/kuba/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1220140 -- '/home/kuba/.local/share/Steam/steamapps/common/SteamLinuxRuntime_soldier'/_v2-entry-point --verb=waitforexitandrun -- '/home/kuba/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun '/games/SteamLibrary/steamapps/common/Cartel Tycoon/Cartel Tycoon Launcher.exe'\0
vs
/bin/sh\0-c\0/home/kuba/.local/share/Steam/ubuntu12_32/reaper SteamLaunch AppId=1220140 -- '/home/kuba/.local/share/Steam/steamapps/common/SteamLinuxRuntime_soldier'/_v2-entry-point --verb=waitforexitandrun -- '/home/kuba/.local/share/Steam/steamapps/common/Proton - Experimental'/proton waitforexitandrun '/games/SteamLibrary/steamapps/common/Cartel Tycoon/Cartel Tycoon Launcher.exe'\0
And unlike launching it directly without the Steam runtime, with it the graphics are correct.
PROTON_LOG=1x1 2022-07
Compatibility Report
System Information
I confirm:
Symptoms
Has to be run directly via its exe, because launcher fails with "EINVAL: invalid argument", which seems to be a Electron issue.
Graphics are messed up, terrain textures are missing or black/white. Everything is shaded dark gray.
Screenshot
Reproduction
Try to run the game.