I HAVE INTERESTING NEWS. I finished the raid today and used the last of my Glimmer to finish off the Invective bounty I had. Once I did, I got the gun from Ikora, went to Xander and it unlocked the missive! Turns out it was A Dubious Task again... Maybe the glitch happens when you get two of the same bounty? The missives are predetermined so that could be what causes it.