Ok, so, i've been having this issue for quite some time, ever since i installed reshade to FFXIV.
More info here:
www.reddit.com/r/ffxiv/comments/bbnhrp/t...nitor_scaling_issue/
At first, i didn't think it was ReShade, so i tried out all sorts of other solutions, to no avail (like fullscreen optimization, DPI settings etc etc.).
However, as soon as i uninstalled ReShade for FFXIV, the issue went away instantly and haven't resurfaced once.
I don't know what triggering/causing it, so i just posting it here so you may take a look at it.