I don't know if this works, but it could be worth trying at least. Try to make a legendary engram drop from an enemy. Once this happens, do NOT pick up the engram! Instead, head back to the tower, and visit the postmaster. Grab the engram from the postmaster, and it will auto-decrypt. See if it registers with your quest. If this doesn't help, then I'm sorry in advance for wasting your time.

What about buying legendary engrams from Rahool?