Hmm, maybe some race condition that sometimes breaks --re-add immediately after ...

Hmm, maybe some race condition that sometimes breaks --re-add immediately after --remove? Might be worth trying separating it into two commands, i.e.:

  mdadm /dev/md0 --fail /dev/sdb1 --remove /dev/sdb1
  mdadm /dev/md0 --re-add /dev/sdb1 --update=no-bbl

I have a simple RAID-1 with metadata version 1.2, too.

My testing was done on Debian unstable (Linux v5.8, mdadm v4.1).