Had this happen in Leveling Duty Finder the other day. We got one of the Endwalker story dungeons and a DPS new to it. Myself (DPS) and my friend (Healer) hadn't run this dungeon in a while, so the last boss got us on a mechanic. It was around 50% at the time. The still standing WAR tank decided to then solo the boss for 10-15+ minutes, taking it down to 1% where they then died, completely wasting all of our time. At least the new player got a chance to participate in the full fight and learn it on the second go. This is absolutely ridiculous and disrespectful of everyone's time. If a tank wants to go solo a dungeon, there are ways to do that without wasting the time of 3 people who registered for a normal leveling run.