This now also happened to me a few times, several days between the occurrences with multiple daily suspend/resume cycles but no full reboot… suddenly the Fn Key stopped working, no PgUp/PgDn/Home/End/Functions Keys like mute/volume/… work anymore showkey didn’t show a keypress when pressing Fn key and when pressing e.g. Fn+F1 showed the same code as when only pressing just F1 … after reboot it still doesnt show a keypress for the Fn key, but Fn+F1 and just F1 show different keycode … same for others … so it seems the Fn key for some reason does not register anymore … also e.g. Fn+Esc does not toggle Fn Lock in this state… i cannot say how to reproduce it, only that rebooting seems to solve it … If you’d like further information next time it happens, i’d be happy to provide those
Ah forgot to mention 12th gen Framework 13 … which i LOVE to use by the way … very happy user 
and full instructions to optimise for Ubuntu followed, linux-docs/Ubuntu22.04LTS-Manual-Setup-12thGen.md at main · FrameworkComputer/linux-docs · GitHub