If I run a game through Steam, I believe it disables desktop effects. After closing the games, the kwin blur behind Windows on the desktop turns black. If I press win + shift + tab to disable desktop effects, there is no longer an issue, but if I re-enable it then I get the same issue. The only fix is to restart lightdm.
I had this problem with a gtx 1070 with closed source driver and now still have it with this amd 570. I am using the video-linux drivers in mhwd.
EDIT: It doesn't only turn black, this time instead of black it's showing a picture of the file manager in the back? Very strange
Normally it looks like this for windows: