There are many possible solutions which all have worked at one point for me.
-Give it time, I've had games like Tales of Arise take a while to open, sometimes steam is doing something else in the background.
-Force close the app and retry.
-Run the game without STL, then retry with STL, make sure to check you are running the same proton that successfully booted the game.
-Try different protons, if you tried 8.X try a 7.X version, etc.
-Try launching the game via Aurora, click on "Launch game," you might need to go to "trainer settings" and add the games executable to path. Note: some exe will work while others will not, Remnant 2 for example. I need to use the exe found in Remnant2>Binaries>Win64 not the one found in remnant2 folder.