Reproduction steps:
1. Open the attached “BugRepro” project
2. Build the project for Windows Platform
3. On Steam Deck, switch to Desktop Mode
4. Connect a keyboard to Steam Deck
5. Transfer the Build to the Steam Deck
6. Add the Build Player executable to Steam as a Non-Steam Game
7. Set it to use Proton Experimental
8. Launch the Player
9. In the upper right corner, observe the “CurrentRes” resolution value
10. Press ALT + ENTER to toggle full-screen mode
11. In the upper right corner, observe the “CurrentRes” resolution value
12. Change screen resolution
13. Observe resolutions

Expected results: In Desktop Mode, when toggling to Fullscreen, the Player sets the resolution to take up all of the screen space (1280 x 800), just like in Gaming Mode. When changing screen resolution, the resolution values are immediately updated in Player, just like in Windows.
Actual results: In Desktop Mode, toggling Player to Fullscreen sets the Player resolution to 800x800, instead of 1280x800. When changin screen resolution, the resolution values stay the same, even after toggling between fullscreen and windowed modes.

Reproduced with: 2021.3.39f1, 2022.3.31f1, 6000.0.4f1

Reproduced on: SteamOS
Not reproduced on: Ubuntu 22.04 + Proton, Windows 10