It isn't a "bug" per se (more of a quirk of how it's coded, it seems), but the graphics settings aren't alterable while Fullscreen is active.
It makes some amount of sense, but if that's not intended, now you know.
Thanks for the heads-up. It's a known issue / somewhat by design. It used to be that you could do that, but when I updated to Godot 4.5 the functionality ended up breaking. The only fix I could find was to disable it. I'll probably try re-enabling it at some point - it could simply be a bug that needs fixing on Godot's side.