Steps to replicate the issue:
- Create a course and two users.
- Enrol one user as a teacher and another one as a student.
- Create a forum activity with enabled whole forum grading (either scale or point).
- Access Gradebook setup page.
- Set the forum activity as hidden.
- As a teacher, grade the student in forum activity.
- As a student, access forum activity.
- Click View grades.
- Confirm, that the grade is visible despite the fact that it was set as hidden.