Comment # 11 on bug 979775 from Fabian Vogt

(In reply to Stakanov Schufter from comment #10)
> So, it seems that the screenlock does not crash, but there is an unpleasant
> side effect. 
> Take a three user environment. 
> Start user A first  session (tty7)
> Start user B second session (tty8)
> Start user B session again (to test)
> This will drop you right away to the open desktop of the second user, but(!)
> that is not all. The system will open an "unused session" in the list. 
> Start now the third user C session. 
> You will log in normally but when you try to use alt+ctl+fn to switch
> between the sessions there you will encounter that:
> A is on tty7
> B is on tty8
> C is on tty 10 (!)
> 
> tty9 is locked, shows a sddm login screen (but default - that is before any
> user logged on) and does not allow to login any user. 
> Thus the tty order is not respected and the user that should be on tty9 is
> on 10 (and you loose one screen as it is locked and unusable). 
> 
> So the solution is IMO sub-optimal because who is landing on that tty will
> think his account is wrong as he gets no error message specific to not being
> able to log in. 

Note that all of ^ is irrelevant as long as bug 1089287 is not fixed.

> What you can note during all this: the user C, when opened after the test of
> "doubling" the session of user B will open in a sluggish mode (takes longer)
> will show shortly the system messages that are normally on tty10 by
> default). Then you have the user opened on tty10, tty9 is locked. tty11 and
> 12 are blacked out, so you do not have the commodity of system messages any
> more.

That'd be a systemd bug.