I learned it accidentally trying to get root on an encrypted dataset working with systemd init without sd-zfs. This turns out to be how the zfs utility works internally to signal the driver “hey it’s okay, I’m a ZFS utility the user isn’t using mount directly”, and how you deal with mounting your root dataset to the temporary /sysroot while having its mountpoint set to / while in initramfs before pivoting root.

Obviously, don’t use that other than recovering your data, if you want to use this array you should figure out the mountpoints properly so ZFS does it automatically. It shouldn’t break anything but it’s gross, either set mointpoint=legacy and use fstab or set its mountpoint in ZFS and use zfs mount.