The thing is that in the libutempter PKGBUILD, there is:
chown root:utmp "$pkgdir/usr/lib/utempter/utempter"
Now the utmp group could have a different GID on the system where it’s built/packaged than on the system where the package is installed. Hence it might have a different group assigned.
There is actually a bug report at Arch as well as a fix, but apparently it was not yet merged. FS#58306 : [libutempter] GID of group utmp auto-assigned via sysusers.d breaks functionality
To get it fixed you can do:
sudo chgrp utmp /usr/lib/utempter/utempter
sudo chmod g+s /usr/lib/utempter/utempter