Recover group memberships after unenrolling and re-enrolling

There is a gap in the very supportive functionality of recovering grades and other stuff after a user has been unenrolled from and re-enrolled to a course.

This is somehow related to MDL-19884 where this issue was already 10 years ago, but I dare to raise it again as there may be solutions or other decisions today.

Steps to reproduce:

• Login as admin
• Create two users S and T
• Create a course C
• Enrol user S into course C with role "Student"
• Enrol user T into course C with role "Editing teacher"

• Login as user T
• Go to course C
• Create a group G
• Add user S to group G

• Login as user S
• Go to course C
• Unenrol yourself from course C (and let's pretend that this was done by mistake)

• Login as user T
• Go to course C
• Go to the participants page
• Manually enrol user S into the course again and make sure that "Retain group memberships after unenrolling/re-enrolling" in the enrolment dialogue is checked

Expected result:

• User S is enrolled again in the course and can use it again as if he had never left
• User S is especially still / again member of group G

Actual result:

• User S is enrolled again in the course and can use it again as if he had never left
• User S is not member of group G anymore