If course has suspended users and "Show only active enrollments" is set to Yes, list of students shows wrong number of students in the list and whole pagination is wrong.

To test this have course with at least 20 students. Make sure that some students are suspended (in my case it enrollment method was disabled). In grader user settings set 5 students per page and "Show only active enrollments" to Yes. In students list not all pages will show 5 students and maybe you will not be able to see all students. Problem is in file /grade/report/grader/lib.php in function load_users. Function first gets correct number of students and then filters inactive students resulting in list that can have less students then you should suspect and can also make pagination worthless as you will not be able to see all your active students. Function should always show right number of students per page.