I have discovered that when in 1080 mode, the output isn’t actually 1920 x 1080; it’s actually 1920 x 1079. The HP monitor may have issues with the non-standard resolution.
I am having issues with my Samsung 2420 not displaying a signal from the mulitviewer in anything other than 1080. Tomorrow I am going to try a few other things to get it to kick over and function on 480. I didn’t have a chance to experiment with other monitors.