Hi,
I have also a problem with my second external monitor (LG W1942TE). Primary monitor Samsung U28R55 connected with DisplayPort works fine.
The LG monitor is connected with a 'DVI-DVI'-cable + a 'HDMI-DVI'-adapter to the Mac Mini M1 2020. Everything was working fine on the Big Sur 11.6.
Look at the attached photo. That's how it looks now.
I would recall that there was the same problem in the past with the first versions of the Big Sur on a new Mac Mini M1 2020.
Any solutions to fix this or do I have to wait for some macOS updates again?