I have Plasma-5 and SDDM under F21. Switching sessions to another (test)
user, also with Plasma, works. Switching back again gives flashing
display garbage and the session is unusable. The problem appears to be
with switching to VT1, not to the content of the sessions as such (i.e.
if I start the sessions in the reverse order the same problem happens).
I have an Nvidia card with the proprietary driver.
I tried reporting this to bugs.kde.org
but there doesn't seem to be a
category for SDDM, which I would think is the culprit.