So, on the 5th time of asking due to glitches with the High Celebrant and the portals, I finally completed the Coup de Grace mission.
Watched the cutscenes, spawned back on the shore... No triumph. No ship. No opportunity to run it again as its locked out on the lure.
Really outdone yourselves on this one Bungie. This mission was an absolute mess.
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Update: I ran it again with one of my alts and the triumph popped. Phew! If that had glitches again I would have been apoplectic... All well that ends well