Tested with
QT_SCREEN_SCALE_FACTORS=1 kcmshell5 kcm_kscreen QT_SCREEN_SCALE_FACTORS=1.5 kcmshell5 kcm_kscreen QT_SCREEN_SCALE_FACTORS=2 kcmshell5 kcm_kscreen
Popup was placed correctly every time. Previously it would appear offset to the bottom right