I looked into this bug today and it seems like this issue has been present in our source code for a while. There is no code handling this case (i.e. marking invite accepted notification as read) at present.
In my opinion “Invite Accepted” notifications should show up as read, like we do with badges notifications. This will be useful in case of “Bulk/Disposable Invites”.
Thoughts @codinghorror / @sam ?