When re-enrolling a previous unenrolled student and requesting that grades are not recovered they are still unable to re-do activities as the activities remember their previous state. The following discussion applies to Quiz but it will apply to other activities as well. This is bad news if you really want a student to re-take the course. This is a common requirement for training courses.

To reproduce...

• Create a course and add a student
• Create a trivial quiz with only one attempt allowed
• as student, access the course, complete the quiz and note that no more attempts are permitted
• as teacher, check the Gradebook and note that the student has completed the quiz and been graded for it
• unenrol the student from the course
• at this point I manually ran the cron a few times in case any 'clean up' jobs are pertinent
• re-enrol the student in the course
• Check the Gradebook and note that they are now shown as having not completed the quiz
• as student, access the course and attempt to complete the quiz. You can't, no more attempts are permitted
• as teacher, note that the Gradebook still shows the student as having no grade for the course

Additional info...
I have been told that this also affects Feedback although I haven't confirmed this. So, it may be unfair to blame the quiz rather than the Gradebook. I also understand that 2.8 may be affected.