I have a second monitor that I use to display a browser window (documentation, chat, etc.) to the left of my main monitor. In Plasma, almost all modal popups are centred on the main monitor (or on the secondary monitor, but that is another bug). However, the user switcher spans both monitors, which is unfortunate as they are not placed as if to be one unified monitor.
Yeah, seen that too.
It was changed into a separate binary in 5.9 which opens a new window for every screen, should be fixed.