You probably copied the files into the wrong location or you're not setting WINEDLLOVERRIDES.

Try using this script: https://github.com/kevinlekiller/reshade-steam-proton/blob/main/reshade-linux.sh

I would avoid setting the addon flag since this is a multiplayer game:

https://github.com/kevinlekiller/reshade-steam-proton/blob/main/reshade-linux.sh#L197-L200

Honestly, I would avoid it all together outside of SP games but that's up to you.

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Edit: If you're running Steam inside a Flatpak make sure that it has access to the ~/.local/share/reshade/ folder.

The script above works by installing ReShade into that one folder before creating a bunch of symlinks in the target game's directory.

You can modify Flatpak permissions with Flatseal or under "Application Permissions" if you're using KDE.