having the same issue as @falkfyrebeard on 6.12.3, I attached my diagnostics and the output of `losetup`.. hopefully it'll help as well? ive had it happen to me the last 2 times ive stopped the array.. 1st time i waitied 20-30 min and then just hard rebooted the server. then just did it again and had the same issue..
root@Excalibur:~# losetup
NAME SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE DIO LOG-SEC
/dev/loop1 0 0 1 1 /boot/bzfirmware 0 512
/dev/loop2 0 0 1 0 /mnt/disk1/system/docker/docker.img 0 512
/dev/loop0 0 0 1 1 /boot/bzmodules 0 512
root@Excalibur:~# /etc/rc.d/rc.docker status
status of dockerd: stopped
root@Excalibur:~# umount /dev/loop2
root@Excalibur:~#
excalibur-diagnostics-20230829-1006.zip