Using kstart --desktop in an KDE autostart command is broken. Applications show up on different desktops than they are told to. To rule out session restoration, it is deactivated, start with blank session. Also, sometimes an application is showing up and can be moved simultaneously on all desktops, when autostarted with --desktop 3 and no further arguments. I can't find any logic by which the desktop is chosen, it seems to be random.