Why are people leaving at Void Vessel boss fight?

I have been running void vessel to get the 50 clears challenge done. I have noticed on more than one occasion someone rushing through the dungeon, not opening any crates or the time extenders, then pulling the whole party to the boss while people are still looting.

Once the party spawns in they up and leave. It’s not like we are having trouble clearing the boss or anything. Does it count as a clear or towards progression on targeted loot if you make it to the boss room and leave? What’s with this?

I’ll note that most groups are courteous and wait at the door for everyone to finish looting, but have seen this a few times now.