Edit: restarting it does seem to fix it. I am not sure how to reproduce it.
I have both one and two monitor setups (see below).
I can verify that it happens on multiple machines, but not on every machine, and not at all times.
Laptop) I have a laptop that I connect via usb-c to the monitor at work - and noticed it first there. I thought it might be because that is a 2 monitor setup (although I have it set to only output to the 2nd monitor, with the laptop screen being black). It does not happen if I detach the PC from the monitor (no reboot or anything - just detach and region screenshot works normally).
Main PC) This had never happened (and still does not happen) on my main computer, which I have connected to the same model monitor at home via HDMI cable. This is a single monitor setup. The resolution is 3840 x 1200, BTW. I used to have a 3 monitor setup, which I replaced with this single monitor, and screenshot captor ran without a hitch on the 3 monitor setup, as well.
New PC) However, I am currently setting up a new computer and have that connected to the same monitor at home via display port (I just toggle the input source). This is the same setup as the Main PC above, but the white screen with small desktop in the upper left corner thing happens just like the laptop at when hooked up to the external monitor at work. There isn't much difference between the new PC and the main PC in terms of setup, so I can't say why it would happen on one and not the other.