Previous and Next buttons become unstable once learner uses menu to jump around
I'm hoping someone can help with this mystery. I've published a course as HTML5/Flash fallback, with menu set to "free" (no restrictions), TinCanAPI, uploaded into an LMS. During QC, we noticed that if the learner navigates linearly, just using PREV and NEXT buttons, no problem with slide chronology. Once they jump around using the menu, and then immediately click PREV or NEXT, it's inconsistent. It jumps anywhere, illogically (forward using PREV occasionally, and backward using NEXT). Only on the first try after jumping to a slide using menu. I can't find other discussions about this specific problem. Does anyone have any suggestions? Thanks so much in advance.