Well the achievement didn't pop up, so i decide to start from chapter 1 in easy and follow the guide step by step and seeing the statues, and the achievement pop up at chapter 9, so... i guess it counts the treasures and keeps it as long you finish the chapter.
If you do a whole chapter opening all treasures and then do it again opening just some of them, i guess it doesn't save the last run but the run where you open everything, it work like that to me.
Some times you may not realize but you only miss 1 treasure like i miss the treasure at begin of chapter 9.
If you want that achievement start again from chapter 1 with calm and do the chest 1 by 1.
Good luck and sorry for the bad english