Somewhere late in the Proton 5.x cycle (I think it was 5.13) Valve introduced Steam Runtime version 2: Soldier, which uses bubblewrap containers to solve various problems with host system libraries, but also broke some things. For example, it wants a newish version of bwrap and no longer works inside most other containers (except for Flatpak, which added special functionality to accommodate it).
If you’re not using your own containers, I imagine upgrading to a recent version of Linux Mint might help, by giving you a newer version of bubblewrap, and/or enabling user namespaces (which bwrap uses) if they weren’t already enabled.
Alternatively, if you’re comfortable running code from Flathub, you might consider using the Steam flatpak instead of your distro’s native package.
Once upon a time, Flathub also had community-built versions of Proton that were stripped of their container functionality. I don’t know if those are still maintained.