Photoshop: GPU not actually working (multiple card issue)

I'm not 100% this is the right place for bug reports so please tell me if I should post this somewhere else instead, thanks.

Photoshop detects my GPU correctly but only uses it if i create a file on the screen plugged to the graphics card.
So for example if I create or open a file on screen 1 and move it to screen 2, screen 2 will be using GPU even though it's plugged to the motherboard. However if I open a file with Photoshop being in screen 2, it'll be using CPU no matter where we move photoshop.
So some tabs/files will work on GPU and others on CPU, even though GPU usage is checked!

If this is unclear I tried to make a quick video to detail this here :  https://youtu.be/Wo-pQ9NaIag
It might be a little subtle, so maybe turn on 1080p to see how pixelated the line actually is.
Also I'm on windows 8.1.