This is an issue I've been messing around with for the past month or two since replacing my primary monitor with a 4K display. Previously when using my 1080 secondary monitor with a 1080 primary display, using the second monitor worked as a playback video device flawlessly.
However, now that I've replaced my primary with a 4K display, it seems that choosing the 1080 secondary as a playback video device results in the video output floating on the primary monitor.
Screenshot below shows the strange behavior as seen on the primary monitor and even if it appears that the video is bleeding off onto the second monitor, it's not. There's nothing on the second monitor.
I've tried changing Window's scaling to be 150% (to match the 4K screen) as some Windows applications seems to struggle with dynamic scaling but it didn't seem to change anything.
OS: Windows 10 Build 1803
Primary Display: 3840 x 2160
Secondary Display 1920 x 1080 (Trying to use as a Playback Video Device)
Tertiary Display 1080 x 1920 (In portrait mode and on but not selected in Premiere for output)