Replying should probably mark notifications for parent posts as read (maybe it already does so and it was just a page I needed to reload)
What I mean is: I was browsing forums, saw your reply, replied to it, then noticed I had a notification, and it was for the post I had just replied to.
I'm not saying a notification (me replying to you) should mark another (you replying to me in the first place) as read, it could be marked as read when you view the thread or something.