We noticed this after upgrading from 1.6.4+ (2006050540) to 1.8.4+ (2007021541),
might be related or not, yet to confirm.

Problem: Student can not unenroll themselves from courses
student role is created with 'moodle/role:unassignself' as 'Not set',
thus unenroll link from admin block or profile page is not shown nor
a student can unenroll at all.

Looking closely seems that this is defined as the default for student role,
it happened as well with new courses and new students created under
version 1.8.4+.

Solution: set 'moodle/role:unassignself' capability as 'Allow'.