First of all, evil developers putting the chest in the center of a lava lake! When I crossed the stones, the lava killed me and there was a resurrection Ankh right there. But after opening the chest I died again and the Ankh was gone. Since I was dead, I couldn't use the rope to climb up and I waited for the remaining time to pass before being resurrected. When the treasure time was over, I still dead, I appeared at the entrance to Sunless Barrens and a few seconds later I was resurrected. Two suggestions: instead of some stones, how about a small bridge or a rope? and the Ankh should not disappear, there is a good chance that unskilled people like me will die when crossing back.
Yes i talked to DC about this last week. Several have had this happen. It hurts to lose that treasure chest. And not expecting this is a bit jaunting for some. Hope you will be able to get another and go for another shot at it. On another note i thought the whole scenario was ingenious though. Kinda hidden down in there. This one tends to borderline Decrepit /bloodstained. This is one that will be fun to see if can be implemented in future maps.