Hi I was having this problem with my 2nd monitor (27" Class 4K UHD IPS LED Monitor with VESA DisplayHDR 400) that I use for work. After updating to Big Sur, I was no longer able to drag documents to the 'second desktop' on my monitor and could not access windows that I pulled to the monitor's desktop if I was to unplug and plug in the usb-c -- they would simply disappear off screen but if you pressed the task manager button they would appear as there. I found that after updating to Big Sur my 2017 MacBook Pro was forcing my monitor to use HDR which I discovered after noticing the irritating colors. After going into the display setting and turning off this option it seemed to fix the problem. Hopefully this helps others!