fstab mounts inside zfs filesystem

You cannot use fstab to hang external volumes off a ZFS filesystem. Entries in fstab are processed before ZFS is available and block the ZFS mount as the directory is no longer empty. This is a race condition unfortunately. 

You can delay the mount by adding a _netdev option to the external volume so the mount action waits a bit until the network is up. By this time, the ZFS volume should be mounted. Not a direct fix, but it works

# ZFS Volume mounts as /srv/media. External volume mounts down further in the tree. Delay until /srv/media is available using _netdev
UUID=effec574-2567-4fe5-969f-135e1822f94a /srv/media/video/movies.1 btrfs defaults,noatime,nofail,_netdev 0 0