I had two students ask to retake a quiz today. I have it set up on the quiz settings to "enable build on last attempt" and, according to the Instructor Guide (and the information the students were told on their screen when they accessed the quiz), the questions that were answered correctly on the first attempt would be hidden. This was not the case today: they were going to have to retake all the questions. 

This is the second feature in Canvas Quizzes that worked last December but now does not (the first broken one I noticed was printing a blank quiz). What is going on??