A handful of UWP apps (Netflix and Edge come to mind off the top of my head) will occasionally get stuck in a state where they won't properly maximize to just one display. I think it usually happens after fully maximizing them (shift+maximize), but this doesn't always trigger it. After getting stuck in this state, I can't seem to find any way to get it out (short of closing the app entirely). I think it's limited to UWP apps, but I'm not certain.