'Pass' and 'fail' slide nav keeps changing

Bit of a weird one this.  Published a course for a pilot group and thought all was well.  But then the feedback comes in that quiz slides are taking the learners to completely the wrong slides regardless of whether they've passed or failed the quiz.  So I had a look at the properties for each QM slide and noticed that they do indeed seem to 'choose' a slide maybe 3-4 slides back instead of the one I select.  I.e. I set 'pass' to go to 44 and 'fail' to go to 44 but minutes later, after doing some other edits, the nav has changed itself to something like 40 and 40.

Is there a bug or some other corrupt aspect that can be causing this?  Am not experiencing it in other courses at present.

Thanks