As i tested in Moodle 3.10, after i make groups in a course for participant, when I want as a teacher just mark students work in forums by group filter it shows all students.

Consider 3 teacher work together in a course with 5 groups and 30 students:

• If teacher X, just wants to mark and set grades for forum posts of group A and then separately check grade of students in group B, in grading page for Forum, all students are showed and teacher should go one by one or search the name, so the filter for grouping is not apply/work by group, as Moodle has it in forum view page.

Attached pictures are showing the situation: