When an activity is hidden the corresponding grade item is hidden as well. When the instructor makes the grade item visible, there is no change from the student's perspective. The student still cannot see the grade.

Either the instructor should not be allowed to change the visibility of a grade item for a hidden activity, or the students should be able to view the grades when the grade item is not hidden.

Andrew Davis makes this comment on MDL-15457: "If you want an activity hidden in the course but visible to students in the gradebook (for some reason) you can hide it in the course then go and unhide it in the gradebook."

This is not the case in 2.2.4+