[Linux/Proton] Attempting to launch a sourcemod from Steam doesn't work

[MCAlexisYT]

Game Version

Version 1.7.0

Describe the bug

When you launch a mod from Steam, it apparently feeds the raw Linux file path into the -game parameter without the Z: prefix (see the screenshot). As far as I can tell, that doesn't happen in other Source games.

To Reproduce

1. Create a folder of literally any name in the sourcemods folder.
2. Copy and paste [steam root]/steamapps/common/Portal Revolution/revolution/gameinfo.txt to the previous folder
3. Restart steam
4. Find the "Portal: Revolution" game without an icon, and launch it

Expected behavior

It should just launch

Screenshots

Desktop

OS: Linux Mint

[MCAlexisYT]

Typing -game "Z:/path/to/sourcemod" in the Steam launch options works just fine, so this seems more like an issue on the Steam side of things.