When you complete the T4 Shadowlands duo (on the USPC hero side at least), the scoreboard doesn't automatically appear to end the instance. Therefore, whoever is not the last person to leave will be given a deserter penalty. I have even queued into a Shadowland duo where it was already completed but the other person would not leave, so I had to get the penalty.
Got my partner to leave before me. Still no feat credit for the duo. Implies no completion. Still, getting 2 (or 3) Marks in a few minutes is worthwhile.
