Corrupted course presentation

So we've had a report from one of our users that a course presentation they've created has become corrupted. As soon as they enter into editing mode, they're unable to navigate past the first slide. Likewise, when viewing the presentation, they're only able to see the first 6 (of 24) slides.

This occurred in our Wordpress instance of H5P, but we've also tested it in the Moodle plugin and on H5P.org with the same results.

Any help would be greatly appreciated.