The Grader Report does not filter activities by group. The problem is presented clearly in this thread https://moodle.org/mod/forum/discuss.php?d=192546

Steps:
1. In a course set up 2 groupings and have a different group enrolled to each of them
2. Set up two tasks/activities, one task for each grouping and set each task to seperate groups (or force seperate groups in the course admin settngs)
3. View the grader report and use the 'Seperate Groups' toggle. You would expect to see the tasks that are relevant for each grouping. The problem is that you don't, there is no filtering happening.

To summarise when a course is using 'separate groups' and activities are assigned to different groups, the Grader Report does not filter correctly, when using the 'Separate Groups' toggle, to show only the activities assigned to each group. Instead the teacher will see all the activities across all the groups in the course.

This means the Grader report fails to provide meaningful and clear information to a class group teacher and impacts severely on the reporting of grouped activities.

As well as a functional issue, this could become a privacy issue across larger courses with multiple teachers grading multiple groups.

Assigning a teacher to a group and prohibiting the permission 'Access all groups' to the teacher role does not improve the filtering.

The group selector also fails to filter when you enrol a Teacher to multiple Groups with the 'Access All Groups' permission prohibited. They still see all the activities in the course (just the list of students changes).

The only way to get a Grader Report listing for one group and their activities (and only their activities) is to create a Course for every group, which goes against all the reasons for having groups in the first place, but one I've had to embark on.

Bug reproduced on demo.moodle.net