Have done 3 weeks of ascendant challenges (18 different challenges in total) on all three characters and still have not received my triumph completion. Have completed the encounters solo, looted the chest, and exited through the portal each time. Also completed the weekly ascendant quest from Petra just in case, yet still no completion. I know this is a known issue for multiple players out there. Just trying to keep the topic fresh so Bungie may see it and address it eventually.
If I'm not mistaken, the Ascendant Champion triumph requires you to complete each unique Ascendant Challenges at least once. It's not counting how many times you have completed an Ascendant Challenge.