IIRC Arch has two Steam runtimes available, Runtime and Native. I can't remember which, but one of them forces Arch to use libraries provided by Arch, whereas the other allows Steam to use the runtime libraries provided by Valve. This could be the root of the problem on Arch if you are defaulting to the one that doesn't use the Valve provided libraries. Because in that instance, it would be safe to assume that the Proton libraries aren't getting used.