A couple of bugs with bookmarks were introduced to Readium 1 by SR2 integration (in this commit):

• When new bookmarks are created after the beginning of a chapter, they go to the beginning of the chapter (and are labeled as being 0% through the chapter).
• When saved bookmarks from older versions of the app are viewed in the latest version, they are labeled as being 0% through the chapter, even when the actual location is not at the beginning.