Forgot to pick up the quest this week, still could do Pantheon W3. Did the 1st encounters on plat, then picked up the quest. First two encounters were checked in the Triumphs, still didn't count towards the bounty.
Who the f* codes such complete and utter bullsh*t?!?!?!?!
You could just track the progress via the triumphs and grant access to the special rewards at the end of a successful run!
/facepalm