Back in time (one or two, maybe three at most, months ago), when I was still mining, I had an AMD rig and an Nvidia rig for those you do not know me, and I encountered weird issues with my Nvidia rig. When I did not let the VGA cable plugged into it and let it like that for some time, I do not know the exact, the screen got resized for a bigger display, so all the screen was visible. The taskbar and the right of the screen was not seenable. Has anyone encountered this issue, and resolved it ?
So your desktop was going into extended mode ? (like stretching on multiple display).
Also to note that, when you have no display connected to your main GPU if you take control remotely of your rig (vnc/teamviewer) you will not be able to have more than 1024x768 4/3 ratio, if you want bigger, you will need to either connect a display physically or use a screen emulator (little HDMI dongle) at least on windows, i know there is a EDID trick that you can do on linux for headless setups.