Steps to replicate the issue:

1. Create a course and two users.
2. Enrol one user as a teacher and another one as a student.
3. Create a forum activity with enabled whole forum grading (either scale or point).
4. Access Gradebook setup page.
5. Set the forum activity as hidden.
6. As a teacher, grade the student in forum activity.
7. As a student, access forum activity.
8. Click View grades.
9. Confirm, that the grade is visible despite the fact that it was set as hidden.