I attempted to solo the new dungeon on my hunter. I did not do it in one sitting and left the dungeon a few times as I didn't have a lot of time to do it in one run. Beat the final boss today morning and my triumph didn't count as complete. Shattered throne didn't need the solo to be completed in a single instance so I assumed this would be the same. If it requires it to be completed in a single instance maybe add the text "without returning to orbit" in the triumph description next time. I hope you look into this and activate it retroactively. Thank you Bungie