Booted from Instance, couldn't loot quest item

I was in a Deadmines group that kept losing people and having to find new members. Eventually we finished the dungeon, but I died at VC. When I got back to the boat, my party all left the group and a message popped up saying I wasn't part of the instance group and I would be kicked out of the instance. I was the party leader before everyone left, and was 1 of 2 people in the group the entire time, so I don't know why this happened. I was able to make it back to the boss, but I was unable to loot his head or the letter because the game thought I wasn't part of that instance, causing me to waste nearly 4 hours.