How to reproduce:

• Ensure that user has “access all groups” role permission.
• Create 2 courses. Add the user to a group in first course but do not add user to a group in second course.
• In course where user belongs to group, create a group due date for an assignment.
• Note that this group date does NOT show in calendar when logged in as user.
• Also note that the situation is fixed (due date event appears in the user's calendar) if a user is unenrolled from the course where they have no groups, or if they are assigned a group there.