Having a simillar issue here but on win 10.
My main monitor is in 2k while my secondary runs 1080p vertically.
In my case, adjusting my main monitor's resolution to any other options does not do anything. It still shows half box on my second.
If I change the second monitor's resolution or even set it horizontally the notification just wont pop on both monitor. Which make sense becasue setting it horizontally means setting the resolution to 1920*1080 instead of 1080*1920.
Just created a support ticket and hope for a good result.
