system Fedora 25
VGA-0 connected primary 1920x1200+0+0
open delete dialogue on dolphin and resolution changes to 640x480
xrandr then does shows 1920x1200 but says unavailable
logout reset X and login needed to reset display
kwin will only use 640x480 (bug submitted at KDE-376512
xfwm4 (the window manager) has nothing to do with this, this is a KDE issue.