Quizzes set so "each attempt builds on the last" are regraded incorrectly.

Steps to recreate regrading problem:
-As instructor, create a quiz with the setting Question Behavior > Show more... > Each attempt builds on the last, set to Yes
-Add two questions, I used T/F
-As a student, take the quiz getting one wrong and one right
-Take the quiz again and correct the wrong answer
-Submit and note the perfect score for the second attempt
-As instructor, regrade the quiz
-The second attempt grade is now wrong and shows that the answer left unchanged in the second attempt is no longer there
-Review the second attempt and note the answer that was unchanged is no longer there