I can only guess, but if you are still connected to a monitor >FullHD communications might get a bit confused.

There is that chip that could do those higher res, but it isn't wired up to use them and there is a monitor reporting those higher res or even total nonsense....

Options:
- Test with another monitor

- When in Mode edit hit "test" on something FullHD or higher

- Modify a mode that is shown up in Monitor to FullHD. Thats how I way back in MorphOS1.4 I created a 1920x1200 mode when all that was on offer were 4:3 modes maxing out at 1600x1200 (or maybe even lower).

[ Edited by Kronos 07.01.2021 - 19:54 ]