It’s a bit of a read but it sounds very much like the issue you are experiencing:
In -NG we allowed GUI to run at native resolution, but it introduced issues with some devices. So now the default is like it was in the past, and the user can opt into running the GUI without scaling, depending on the hardware they have and how it handles it.
From: Gui at 3840 x 2160
To test if it is the display resolution causing the issue, you could try this:
SSH into the box as root
vi /flash/config.ini
find HDMI custom mode and remove the hashtag from modeline=...
save file and exit [ESC] :wq
reboot now
exit