I've seen the same issue on the new Plugable Pro 8 dock with a similar configuration as cmergel. I am using the external monitor as my only display and I have unchecked "Allow screen to auto-rotate" in the screen resolution properties. Any UAC prompt causes my display to change to Portrait (flipped).
My workaround was to disable the "Switch to Secure Desktop When Prompting for Elevation" policy setting as documented
here
Once disabled, UAC prompt no longer auto-rotates the display. The screen does not dim when the UAC prompt appears, but it's an acceptable alternative to disabling UAC completely IMO.