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