Not saying this is what happened, but check your vault inventory. I had SOMEHOW managed to get an exotic engram into my vault, and upon completing a Xur quest it kept me from receiving the cypher, as you can have only one at a time. Sorry to hear you put in the work and got slighted, in any case.
Thank you for your help, but it still seems like i can't find it if i did get one. I just double checked all my characters inventory,vault, and postmaster but still can't seem to find it. If you need any proof, like photos or video i would be glad to send it, even though i cant prove that i completed the quest by photo or video.
Oh Im just a lowly forum member, offering advice with what happened in my scenario. If a Mentor or Employee hops in here they might be able to help you further. Wishing you the best!