My system was working great for 131 days straight and then I powered down the workstation for a planned power outage. When I powered back up, the right hand full screen display monitor is 180 degrees out of synch with the Premiere program window now. It shows the desktop when Premiere is up and it shows the program content when Premiere is minimized.
After Effects works fine and uses the 2nd monitor like it should.
I suspect the problem arose because on reboot, Windows 10 reassigned monitor IDs... my main display USED to be display #1 and my full screen video monitor was Display #2.
Now it's reversed.. main display is #2 and full screen output display is #1. I tried switching off the right hand display and that renumbered the main display to #1, but as soon as I switch the 2nd monitor on, it steals #1's ID again. When it's like that, Adobe Premiere can't display output on the 2nd monitor.