I followed the instructions and more to get 1080p to remain available at
L4T-README file README-vnc.txt
It caused the HDMI monitor to become 1280 800 resolution.
It also would not allow 1920x1080 to be selected.
I research and found the fix and have attached xorg.conf (1.2 KB) .
I use VNC on a VGA remote so the SCREEN ID is “Default vga Screen”
so when I use VNC I set screen using system settings to 1024x768.
I found this after extensive experimenting after reading this NVidia link