Reappearing notifications

I have a constant notification, a little red '1' on the bell icon. It's telling me there have been new posts on a thread I contributed to yesterday.

1. When I click on it, it takes me to the first comment after my first post on the thread, despite the fact that there are three more pages of comments and I posted again later on the thread.
2. When I try to get rid of it by clicking 'mark all read' it goes away but reappears when I navigate to any other page on boards.
3. I have unfollowed the thread, and a few others, but it won't go away.

I'm always very quick to suspect user error but in this instance I don't know what I could be doing wrong. The only thing I can think of is that put one poster on 'ignore' so I haven't 'seen' those posts on the thread. That seems like an unlikely explanation though.