Option "--recursive" does not seem to cover datasets with legacy mountpoints

[m-ueberall]

I saw that exec zfs-auto-snapshot --quiet --syslog --label=hourly --keep=24 --recursive tank/var [...] created snapshots for both tank/var and all sub datasets/filesystems but the one ("tank/var/www") with property "mountpoint" set to "legacy".
The man page does not mention any exceptions with respect to --recursive, however. Did I overlook something or is this a bug?
(The machines in question run on Ubuntu 16.04 LTS, kernel 4.15.0-43-generic, zfs/spl packages v0.6.5.6-0ubuntu26; explicitly adding the above "tank/var/www" to the list of filesystems on the command line yields the (expected) snapshots.)