Multi window not working correctly

[Shadyfurball]

I'm having problems with multi window/split screen. When enabling it the screen stays black. If I move the window around I can see the content in the upper left. Screen screen attachment for what I mean.

Any ideas how to fix this?

I've tried ticking the developer option multi window resizing on and off and still no dice.

Rooted with magisk and one android version 7.

Thanks