This might be because kwin picks up the wrong dri device. I believe I have experienced the same issue playing with different configurations. As you can see in my post in nvidia’s forum, it seems like my PC is hallucinating 
You may want playing with the KWIN_DRM_DEVICES environmental variable, something like
KWIN_DRM_DEVICES=/dev/dri/card1:/dev/dri/card0
and see if there is any change. I doubt but it’s worth trying