Went flawless with my team and I did not get the triumph for the completion. You can see my Warlocks game history and on raid report here: https://raid.report/pgcr/7360365468
Bungie game history: https://www.bungie.net/en/PGCR/7360365468?character=2305843009319875186
Did you start at a checkpoint by any chance? Even if that checkpoint was the very beginning, that may have been enough to have negated a flawless run. You need to start clean, with no checkpoint, and you can't leave at any point until the raid is completed for it to count.