Another year and still a bug
Yup. Still not fixed. Awesome... Even when I got to the end and there were both stacks they didn't complete the instance.
If you miss any then you end up passing one way drops and need to start over by leaving the instance
Leave Instance>Try again...
First time, I had missed a stack down a hole just past the entrance. Once you find them all you can Milestone, leave instance or whatever out to go see Broin.
Figured out how to finish the instance
I had this problem too, and it is confusing, but it at least does not seemed bugged at this point. I found this thread because I didn't know what to do, and then figured that I must have missed one of the stacks of paper, which I had. It would probably be helpful if it listed how many stacks of papers you needed.
I thought I couldn't go back and get the stack I'd missed, because I had already jumped down. But it turns out you *can* jump back up to the upper level. You just need to find the place with a pile of rocks and jump from rock to rock to get back to the platform above. I did this with relatively little effort (and I'm not a good platform jumper :P) and then ran back toward the entrance and "deleted around" until it selected the last stack of papers I needed. It sounds like most everyone missed the same one, down in a recess with a sleeping goblin.
When I picked up that last stack, it gave me a different message and said Instance: Complete. I didn't need to restart the instance or anything. Hope this helps!