Was doing a flawless CoS. Got to Galrhan phase 2. Went fine then we got to DPS phase. Partner and I went to switch buff. I had 30 seconds left on buff. We both shot witches vessel. Vessel did not despawn. No buff swap happened. My partner was killed by the architects 2 seconds later. Good to know flawless CoS is still bugged.
I got it done using the "work-around" mechanic. It's actually not too bad, just practice with a team before attempting it.