I hope I can explain the issue with my poor English.
We can choose groups for forum on the forum/view.php. On the forum post page, we can see the which group is selected. But in some course formats (social format) we can directly post to forum on the course main page without seeing forum/view.php. Thus user can't see the group selection dropdown. Group selection dorpdown should be on forum/post.php. It makes more sense.