Internet cut out as I was descending into the treasure area on PoE. >_>
This is so lame. Even the 30 second limit when a single person opens a chest, stinks.
They could have a boolean that is true or false based on the following condition:
When a person kills the boss, the treasureEarnedBool = 1.
Once they grab the treasure, it's set to 0. However they distinguish it between each PoE can be done in different ways and they put badly already have a way (or in lame terms, a variable for each e.g. treasure35Earned).
Of course then you can't test whether you've earned the treasure key AFTER killing the boss, so a timestamp mechanism would have to be implemented which can be resource intensive. Just have to convert it to some small int and can be done probably on a minute grain between HoW release date and now to save space.
So if 2 other people already got the chests, for instance. Either they can't join you to go back to the chest room or it won't allow them to open it because their boolean is set to false. I'm guessing they would just stop them up front from going in in the first place but regardless. That'd be the only "checkpoint".
"players in the fireteam don't have access to this mission. Please press Square to start mission fresh" message if you wanted to lose that checkpoint and play the whole PoE again.
Seems like there are ways to fix this but coming from someone who has no idea how they implemented anything.