Eloquent leftJoin retrieving null is overwriting all other values

I am trying to retrieve some "results" from a test in my students' grades database.

It is setup so that a student hasMany groups and groups have many results, then results belongTo a test.

I am trying to retrieve students who are attached to a group, and also any results from a test with id of test_id my queries are:

$students = Student::join('group_student', function ($join) use ($group_id) {
        $join->on('group_student.student_id' , '=', 'students.id')
            ->where('group_id', $group_id);
    })->leftjoin('results', function ($join) use ($test_id) {
      $join->on('results.student_id' , '=', 'students.id')
           ->where('test_id', $test_id);
    })->orderBy('last_name', 'asc')->get();

problem is that if there a results with an mark of null it works otherwise it returns null for important things like student_id.

Why does eloquent do this? how do I stop it from doing this again?

Any input appreciated!