We discovered the quiz originally had 17 questions. Three students started a version, then the instructor cut #16 from the list. All other students took a 16-question quiz and things worked as expected for them. But with the first three students, there was the issue originally described.
Short term, I added back the original #16 to the main quiz set details. Now we can visit the quiz version for all students in the class. For some, we see a 17-question 1uiz, for others 16. It's enough to get what the instructor needs, and no new people will be taking the quiz.
But this goes along with some other recent observations, that there are some attributes of a parent quiz that are inappopriately being used while working with a quiz *version* for a student. In this case, trying to view these student's quizzes should have just let us see all 17 questions and responses, since that is how this quiz version existed for them in the database. But the fact that the parent quiz was down to only having 16 questions somehow caused the issue.