This cannot be unless you modified your setup, whenever /storage is mounted it must happen somewhere on boot, per default /etc/systemd/system/storage.mount.
If you setup mirror, that file is changed on the mirror so the partition is not mounted from /dev/sda but from /dev/mmcblk0 if you boot from that mirror SD. Check /etc/systemd/system/storage.mount on the mirror disk.
If that is not there you have messed up something I guess, possibly because you changed the storage at some point in time. Or any other reason out of many many possibilities we won’t find out.
Find out where the mount happens, remove it, mirror again.