Scenario:
$ zfs create rpool/p
$ zfs set canmount=noauto rpool/p
$ zfs umount rpool/p
$ zfs create rpool/p/c
$ zfs get -r mounted,canmount rpool/p
NAME             PROPERTY  VALUE     SOURCE
rpool/p    mounted   no        -
rpool/p    canmount  noauto    local
rpool/p/c  mounted   yes       -
rpool/p/c  canmount  on        default
In another shell ensure that rpool/p/c is in use, for example:
$ cd /rpool/p/c
Then:
$ zfs set canmount=off rpool/p
cannot unmount '/rpool/p/c': Device busy
But there is no reason to try to unmount rpool/p/c in this scenario.