The best thing to do is to start by posting your garuda-inxi. That way we would have all the information we need about your hardware, drivers and the exact model of the monitor. That makes it easier to help.
Furthermore, Linux is not Windows - just because something works on Windows doesn’t mean it will work on Linux.
You have an old R9 390 with 1x DisplayPort 1.2 - maybe Linux doesn’t like the fact that your unknown monitor model may have a different DP version set or that 4K doesn’t work properly for some other reason. And obviously it hasn’t worked for over a year…if there is a problem you fix it immediately and don’t wait until it gets worse btw…
Edit: Take a look on this:
This may not be the same, but it gets the idea across for your troubleshooting.