I found a thread from like 5 years ago where this is the case too. I recently got a 1440p monitor, but im running it with a 1080p monitor aswell. I cannot figure out a setup where I can run ESO in 1440p on my main monitor, without it just reverting to 1920x1080 if my 1080p monitor is connected. the only way I've been able to run it in 1440p is if I straight up turn the other monitor off, or if I literally disconnect the cables of the other one, then run ESO on the now single 1440p monitor, then plug the other one in, THEN it works fine, but that is way too much of a hassle to do every time.
Is this just going to be a bug forever? or is there some workaround?