MrBranch thanks for notifying me by thread, couldn't find the thread back when I wanted to respond, so doing it here.
The "quest doesn't unlock" issue has been around for a while now, and doesn't only affect "Ranged Units". I've added this thread in my overview of reported issues thread under nr 3 of "quest doesn't unlock"-issue.
I'm afraid I can't do much more then collect it and hope it will be seen by the devs.