Greetings! According to the API (through [url=https://bray.tech/2/4611686018435452793/2305843009260314759/activity]bray.tech[/url]), your most recent Master Vox Obscuras are shown as incomplete, meaning that you or the fireteam leader may have left too early for the triumph to register as completed, but the seasonal challenge did. Unfortunately, this means that you'll have to run it again and wait for the Mission Complete UI popup.