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.
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Ill try this one... If this or the other guy suggestion doesn't work ill go back to Rainbow Six Siege