I've had a similar problem for the last year, where every time I turn off my 1440p monitor, all the windows reduce to 1080p (or lower it seems) area. It is crazy fucking annoying, but whatever I guess. However, just this week at work we had a guy working from home, and one of his monitors (the displayport one) wouldn't work via remote unless it was actually turned on. Since we just use Anydesk, that means everyone can see what he's doing. Not that he was doing anything he shouldn't be, but obviously it shouldn't work that way.
Has anyone figured this out? I can't remember if mine at home (GTX2080) has 2 HDMI or if I'd have to use DP for one, but his cheapo work computer only has 1 HDMI and 1 DP, so he HAS to use those. I've Googled it in the past and didn't come up with anything, but it seems like it is going to be a much bigger issue now with people working from home. Having it disappear completely just by turning it off is insane and can't be expected behavior.