Just ran Prophecy in a fireteam of 3. I never died. The triumph did not pop. My stat tracker also shows 0 flawless runs. Attached a dungeon report image link for proof. Didn't see this as a known issue.
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If you are in a team, nobody can die to be able to get flawless. If you soloed it then you should be able to get flawless. Also flawless triumphs has been bugged since the update sometime after beyond light came out. The update i'm talking about is when they stopped people from getting flawless after they start the dungeons at the boss checkpoint instead of completing the whole dungeon.