A buddy of mine and myself just did Prison of Elders. The chests were opened, but my buddy died on the fall and had to wait for the respawn timer to go down. But because the chests were open, the match had a 30 second End of Mission timer and he missed out on opening the chests entirely. I believe the Prison of Elders should end as a Raid would, and players leave on their own accord, allowing them to open their chests without having to worry about rushing to get the loot.