A new quiz (just one of three created on the same date) lost most of defined correct answer designations though the points assigned remained.  So, even though the correct answers were selected, the quiz taker was told they failed the quiz.  

Interestingly, when I went back through and added the correct designations again, the individual test pdf downloaded were updated to show the correct score, but the on-screen table of responses doesn't reflect the correct scores, nor does the exported csv.

What caused this? How can it be prevented in the future?