Thank you for your replies!

Does the same happen once you are logged in? Did you try the monitor
on another computer (i.e. e.g. without GRUB)?
Yes, same effect during POST, OS booting, logged in (text mode), and
GUI running.
Connecting the faulty monitor to another machine did not help.

Did you try a different monitor to eliminate any possibility it is GUI
effects from Winders?
If I connect another monitor to my primary machine, everything shows
up fine, so it's definitely a problem with the monitor itself.