I tried capturing it with recordmydesktop and all shows fine on the video so sadly don't have much to show. May even maybe be a kwin bug?
I can not reproduce this with 5.2, so perhaps this is related to kwin or may be fixed in 5.2? Can you still reproduce this?
I can indeed not reproduce this anymore, may have been either a driver or a kwin issue if noone changed anything in systemsettings to fix it.