For those who use Wayland and autostart, don't forget to edit
/etc/sddm.conf.d/kde_settings.conf
from KDE5: Session=plasmawayland
to Plasma6: Session=plasma
to confirm the exact name, check what is available under /usr/share/wayland-sessions/
if you're stuck, try pacman -Q | grep -i kde
and pacman -Q | grep -i plasma
and remove everything related, then fresh install plasma-meta or plasma group and it should work