If i am understanding your question correctly, you are asking why threads you are participating in are not bold when being viewed on a forum list.
If that is the case, the explanation is that threads that contain unread content by you the user will be in bold on the forum list. Once you open/read one, it will show up in regular font until there would be a new, unread reply. Then, it will show back up as bold again. Since you have have already read your content, it will show up in regular font until there is a new reply to it.
Please see attached screenshot from my device as an example.