there seem to be some issues with sddm on wayland, that are already fixed upstream but it didn’t have a release in quite some time so those fixes are not in the repo version.
This could be solved by switching to sddm-git from the AUR for the time being.
I also have shutdown issues like you have on KDE Plasma when using wayland and rebooting/shutting down with systemctl reboot/poweroff. When using KDE’s shutdown/reboot buttons the issue does not appear.
As far as I remember, sway uses generic syscalls, so that should be the same issue.