In the screenshot below you can see beside the 3rd topic from the top there is a red circle indicator with a “1” in it which I assume is telling me that there is 1 unread post in this topic.
However, the first two threads also contain unread posts for my user, but they do not show this icon.
Of note, the first two threads were migrated over from a Vanilla forums software. Maybe that is causing the issue of no indicator of new posts for those threads?