non-steam game resolution issues

Your system information

• Steam client version: 1784778118
• SteamOS version: 3.8.24
• Opted into Steam client beta?: Yes [Yes/No]
• Opted into SteamOS beta?: Yes, but tested on both [Yes/No]
• Have you checked for updates in Settings > System?: Yes [Yes/No]

Please describe your issue in as much detail as possible:

Describe what you expected should happen and what did happen.
I added discord, firefox and spotify (all from the discover tab, all flatpaks) to my steam as non-steam games and expected them to fill my entire monitor while in game mode.
What happened was it would leave black bars on either side of the program in every case, except for discord where the resolution was just terrible but it filled the screen.
Adjusting the resolution lower raised my confidence in saying that it isn't rotating the resolution right but it is rotating the program.
So when using 1920x1080 it would be ~1080 wide and (presumably) 1920 tall.

Steps for reproducing this issue:

1. Install SteamOS on a desktop
2. Add any flatpak app to steam as a non-steam game
3. Go back to game mode and try launching them

Reproduce this on a SteamOS desktop by installing Discord, Firefox, or Spotify from Discover, adding the Flatpak as a non-Steam game, and launching it in Game Mode. Compare the app's orientation, resolution, black bars, and image quality at different display resolutions; done means Flatpak apps fill the monitor without distortion or side bars.