Hello.
I've found this issue recenlty, when I was trying Windows 10 on my Mac Mini 2016. For the record, I wiped out MacOS completely in favour of Linux two years ago.
The point is, it doesn't matter which browser I'm using (Google Chrome/Chromium/Brave...), Linux (currently Debian) only can play Stadia at 720 with ethernet connection and 100mb up/down. 1080 is totally unplayable. I've tried every tip over the Internet, and nothing worked.
So, as I said, I was trying Windows 10, and I was curious, so I've installed the new Edge version, and tried Stadia on it. And to my surprise, I was able to play Stadia at 1080 with absolutely no issues. Great detail and fully playable and enjoyable. The same with other Chromium based browsers.
So, my question. Why is it possible that I can play Stadia at 1080 on Windows, and I can't do the same on Linux, using the same machine? Any tips or something?