This is a bit of an old bug that I've been dealing with for quite some time, and I am not 100% sure on the cause, but have some theories. I am using zfstools 0.3.6_1.
This bug is that old snapshots that were locked from delete (in use) during backups, I presume, prevent the script from cleanup up the snapshot. The problem is that the system never goes back to all of the past ones that it missed to clean them up. The result is that you are left with a bunch of snapshots laying around from years ago, in some cases, for something like an hourly snapshot.
IMHO, this tool should be considered to "own" the entire namespace of "zfs-auto-snap_$timestamp" etc, and thus no harm in going over the entire history to at least clean up older stuff that sticks around an extra moment.
Any ideas if this is a bug or something that can be considered implementing? I know it wouldn't be the first time that our systems ran low on space due to this issue.