Found it! It's actually specific to upgrades. https://github.com/yast/yast-update/blob/15c6f2148e05aaf5cdc77f3620658667f4e222ac/src/modules/RootPart.rb#L807 runs "mount -o bind -t none /run /mnt/run", and in line 825 it runs "mount --bind /run /mnt/run". The duplication doesn't seem intended to me. As /mnt/run is a bind mount, this is effectively "mount --bind /run /run", which hides all mountpoints inside.