Yes. pacman-mirrors fails if the ping domain is down. It is strange because I originally created an explicit loop which would only fail after connecting to three different domains.
Somewhere along the way the first of these three domains has been altered to be http://ping.manjaro.org/pacmanmirrors and the loop would return False when the first domain was offline - so when ping.manjaro.org went dark it affected pacman-mirrors as well..
I have fixed the bug - it will become available as pacman-mirrors v5.2.1..