I have found that the 'reported' emails, once manually deleted, do not appear in the deleted items folder, so hopefully the items are being successfully reported.
I've noticed that as well, perhaps that's the real problem; the report feature tried to delete (move) the email to the deleted folder, but that fails so the email doesn't get deleted (moved) and so stays in the junk folder and reappears after a refresh.
Oh more weird, in the mobile app if I report I can see it in my deleted items, but if I refresh it moves back to the junk. If, however, I deleted the email in the deleted folder (after reporting it for perhaps a 3rd time), and refresh it does not go back to the junk.
Sounds like some very gone wrong code at Microsoft.