We encountered a situation in Moodle 3.11 where multiple students' answers where changed retroactively by the regrading process because the question was changed after the students' attempts. This lead to incorrect grading.

Steps to reproduce the problem

1. Create a quiz.
2. Add a new question of type "Matching".
3. Add a subquestion "Question A" with the answer being "Answer A".
4. Add a subquestion "Question B" with the answer being "Answer B".
5. Add a subquestion "Question C" with the answer being "Answer A".
6. Log in as a student and take the quiz.
7. Choose "Answer A" for all three subquestions.
8. Submit the attempt. Your grade should be 66.67%.
9. Log in as an admin again and edit the question.
10. Swap the first two answers, so it's now "Question A" -> "Answer B", "Question B" -> "Answer A" and "Question C" -> "Answer A".
11. Look at the student's submission. It now says "Answer B" for all three subquestions, but the grade is still 66.67%.
12. Regrade all attempts. The student's answers are still shown as "Answer B", and the grade is now 33.33%.

Obviously, this is bad. You could argue that it's is not a bug, since we changed the answer that the student initially selected to "Answer B" ourselves. But even then, the answer that the student gave for Question C should not have been affected, since we didn't change the correct answer for Question C.

The worst thing about this is that after the regrade, every trace of the answer that the student initially gave is erased from the Moodle database. Even the attempt steps now say that the student selected "Answer B", and they are usually what we refer to when a student claims that they selected a different answer than what Moodle says.

This issue appears to be fixed in Moodle 4.0 because of the question versioning that was introduced.