I think It was labeled as "Not a bug" in your case because of 2 things:

1) (Less important) You mentioned that "Flat GUI" option does nothing, but this option relates only to character inventory screen and not every menu in the game. Yes, I agree that the wording is a bit ambiguous and you could ...

Great!
Thank you for your help!

Sounds great!
Thank you!

Any solution would be great.

Thank you very much for looking into this on such short notice!

Hello,

I've encountered some problems while playing space age DLC. A lot of space\rocket related menus are too large for Steam Deck's default resolution(1280x800) which renders some buttons or item slots inaccessible.

I couldn't find a way to move them but that would not solve the issue, because ...