A large number of users with the exact same problem on this forum post. There is a setting suggested there but it looks like the only confirmed cure is to replace the MB.
Optiplex 7090 MT with two DisplayPort outputs (lets label them A and B) + optional VGA. I have two monitors and am using onboard video (UHD 630). Each monitor works in port B but neither is working with port A. Have tried all combinations of cables and monitors and it seems everything works for port B but not A. Goal is to have two monitors together but cannot seem to get a signal from A in any configuration. There is some connection being made as I hear system tone when I add/remove connection from port A. Also, the monitor connected to B will flicker off/on when I remove the connection to A. Any guidance would be appreciated. Thank you.
Optiplex 7090 MT - one DisplayPort not working