This may be related to
MDL-12305. I am on a quiz, adding questions to it. If I show the reorder tool, change the value of one question, and then click Save Changes, everything works fine. (However, if I hit RETURN and not click the Save Changes button, it never works.)
However, if I change the value of two (or perhaps more) and click Save Changes, not only are the questions not reordered but the two questions disappear from the quiz.