When a quiz from course A is restored into course B multiple times with the restore being done in the quiz page, from the second restore, it creates a dynamic link between two quizzes' set of questions. That is, if you modify the questions in Moodle shell A, the update will be reflected in Moodle shell B.
I have tested this in Moodle Demo using two Mount Orange School courses and above are the steps to reproduce the error, which I followed and documented.
This shouldn't be the expected behaviour as a new instance of the quiz/quiz questions should be created in course B.
I couldn't reproduce this same error when doing the backup/restore in the course main page by going to More > Course reuse. It seems to only happen when the backup/restore is done from the quiz page.