In FreeBSD 12.2-STABLE before r369312, 11.4-STABLE before r369313, 12.2-RELEASE before p4 and 11.4-RELEASE before p8 due to a race condition in the jail_remove(2) implementation, it may fail to kill some of the processes.
https://security.netapp.com/advisory/ntap-20210423-0006/
https://security.FreeBSD.org/advisories/FreeBSD-SA-21:04.jail_remove.asc