unable to properly change screen resolution

with both optifine, as well as the vanilla versions of 1.16.1 and 1.15.2, when I try to change my screen resolution to 1920 by 1080p, it just stretches the pixels out, giving the game a greyish hue. I have a 1920 by 1200 monitor, so it should give the game black bars at the top and bottom of the screen, but it doesn't. for some reason, this doesn't show up on screenshots, so I had to take pictures with my phone, however, it isn't very noticeable.