I had the same problem on my BananaPI.
Thank you andre. Downgrading systemd fixed it.
It seems that systemd fixed it in version 231-6
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=832713#331but arch is currently distributing 231-3.
So the only way to quickly fix it now, is manually downgrade to an older version (230 or less) that you can find in /var/cache/pacman/pkg, and remember to do it at every system update