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That's a mistake -- from recollection on some of the activities which are setup with sub-sub menu pages, I may have removed the next previous arrows because they become confusing when the activity sub-page is pulled into an external environment like a learning management system.
By design, things in the 'navigation' class (including the Previous/Next arrows) are not supposed to render when using the IFrame hack or when exporting to an LMS package.
If they do, please file a bug report. :-)