SDDM still is only experimental Wayland support and I've seen SDDM keyboard bugs like that. I may be an X11 dependency it requires is missing (that would be an archinstall bug). Since Archwiki now recommends to KDE default on the plasma login manager, I'd second @f33dm3bits suggestion to use that.
You can try without re-installing, switch to a terminal (
You can try without re-installing, switch to a terminal (
ctrl-alt-F3), login, disable SDDM (systemctl disable --now sddm), install plasma login manager and start it (systemctl start plasmalogin.service). If it works, enable it. You can later switch login-managers once you are settled with the desktop, if you still wish to. If it does not start, it may be graphics related (which I doubt, since SDDM starts).





