I am experiencing this on https://moodle.org/mod/forum/view.php?id=8149

Steps to reproduce (you will need to switch roles a lot. I suggest logging in as Student and Teacher in two different browsers):

1. As student verify that you have unread forum post tracking enabled.
2. As teacher, create a blog-style forum in a course.
3. Start a discussion there.
4. As student, go an look at the forum. (You can see that there is a new unread thread.)
5. As teacher, go and reply to the discussion you started.
6. As student, go back to the course page. You can see the post is unread.
7. Click into the forum. Note you cannot see which thread has the unread posts in it. (Well, in this case there is only one thread, but if there are many threads, it is a problem.)
8. Click Discuss this topic. Now you can see which post is unread.

The best way I can think of to fix this, is to change the '(1 reply so far)' mesage in the UI to '(1 reply so far, 1 unread)', with the 1 unread bit styled like the '1 unread post' message on the course page.