I have a new Asus VP28UQG monitor, it has been working absolutely fine (except won’t display output from my SP3), today it has developed a weird blue tint over a 3rd of the screen.
If with using 2 different connection and sources to the monitor, you still get the same fault, it would seem the monitor is the issue. You could try unplugging it for a minute (power cord) and see if that helps. Sounds like it is defective tho, and being new, it should be returned