I discovered this fix for Clash Royale (might work for other apps also). I am currently on Oxygen OS version 9.0.3 but it should work for any version of Oxygen OS!
1. Uninstall Clash Royale
2. go to APK mirror and download Clash Royale 2.1.7
3. Install the downloaded APK
4. Go into setting and you should now see it as an option to make Clash Full screen
5. Lastly update clash via the google play store and that's it! It should work now!!