Okay, let's try this. When you open a converstation, at the far left are the participants - you and whomever has been corresponding with you for that particular subject. Just below that list is a box that has a bell at the far left, and will say "Turn notifications off". As far as I can tell, if you click that it should turn off notifications for just that converstation. You might try that, then clear your cache. Do it for each converstation (or in your Profile, turn off notifications for all PMs) and see if the notices disappear. If so, then try re-enabling them and see if the notifications reappear for viewed conversations.