This explains the failure - but the mystery still stands - odd name
On my system the sudo package is newer - catering for the unstable branch
/usr/lib/sudo/sudoers.so is owned by sudo 1.9.8.p2-3
Checking with branch-compare - the version of sudo on all branches is 1.9.8.p2-3
Switch to root
su
Then run
pacman-mirrors --continent && pacman -Syyu