To ensure games run well on Linux either via Native Linux builds or Windows games with Proton, part of the magic is in the Steam Linux Runtime. A new version of it, the Steam Linux Runtime 4.0 was recently put up with some pretty big changes.
What’s the point of it? It ensures Steam and games run through Steam on Linux work properly across all the many different Linux distributions. Another secret Valve sauce for Linux. Well, not secret at all but you get my meaning I’m sure.
It doesn’t work fine out of the box. I tried it on Opensuse MicroOS a year and a bit ago and had to search 3-5 pretty undocumented solutions to big problems before being able to play the same games that non-flatpak could.
Out of the box, proton didn’t work at all.
Sometimes you have to allow access to some things outside of the Flatpak container. I use Flatseal for that.
I don’t think flatseal can set the background permission, but I might not recall correctly: