Ok. Not sure why this is wonky. Here are 3 suggestions:
1) Download the latest version and re-export:
2) Try a different page format (e.g., Slider--it would look different in the multifile format, but maybe it will change something under the hood).
3) Add this code as a .js file in the Resources section of the in5 dialog:
$('nav:hidden, nav #backBtn, nav #nextBtn').show();
if(nav.current == 1) $('nav #backBtn').hide();
if(nav.current == nav.numPages) $('nav #nextBtn').hide();
It's actually already in place, but maybe calling it twice will help.