U2518D, Nintendo Switch, capped at 780p?

I have my Nintendo switch system connected to my Dell U2518D monitor (HDMI out to HDMI in). The system connects and functions fine with the exception that the graphical quality is 780p. I am confused as to why as the Nintendo switch has 1080p capacity (and it's set to that). My PS4 and computer function at 1080p on my U2518D as well. And when my Nintendo Switch is connected to my television it is also at 1080p.

Thanks