I didn't test with a reboot, an array stop or neither a docker/vm stop. I can suppose was some dockers that uses this resource.
But I've solved with this without shutting down anything:
root@littleboy:~# zfs set mountpoint=none disk3/backup
root@littleboy:~# zfs destroy -vr disk3/backup
will destroy disk3/backup@littleboy_2023-12-19-04:40:40
will destroy disk3/backup@littleboy_2024-01-07-04:41:17
will destroy disk3/backup@littleboy_2024-01-09-04:41:07
will destroy disk3/backup@littleboy_2023-12-21-04:41:06
will destroy disk3/backup@littleboy_2023-12-22-04:41:06
will destroy disk3/backup@littleboy_2024-01-05-04:41:21
will destroy disk3/backup
root@littleboy:~#