On 11.08.2021 the forum was updated from vbulletin 4.x to 5.x. The counter for the already read posts was not taken over, because the search works completely different.
But you can easily mark all posts (channels) as read. Then, when you click on "new topics", only the not yet read posts will appear.
To do this, go to the bottom of the main forum overview and click on the link "Mark all channels as read".