Hello,
I think this is happens when a user self delete the topic (OP).
I created a topic with a non-staff (TL1) user and than delete it with this user. This user can undelete the topic OP with clicking the 
icon.
But in this situation staff users can’t see the undelete icon. And I think this is because the user decided to delete the topic and there is no reply yet so user free to delete it.
There is only a wrench button which is also not contains undelete…
And the topic actions buttons also not contains.
If this is the case I think the only way to undelete the topic OP from staff side is open the topic and revert the topic history.