The 'View post since last visit' link has always been specifically for displaying new posts since your last visit to the board, as was the same for the new posts forum icons on the forum index page. Whether you read the topics and posts or not, they would show as being read on your next visit, so this function still works as expected.
You're probably noticing that the forum new post icons are still showing that there's new posts in the forums, even though no results are being returned by the 'View post since last visit' link. The reason for this is because of an update that was done last month, which makes forum posts stay as unread for as long as you haven't read it. Which is like what many forums have, except better in that it only marks up to the posts on the page you viewed as read. You can view the the list of unread topics by using the link right below the last visit link, titled 'View unread posts'.
I have yet to finish some of the user customizable features and changes for this update however, so while functional.. It is still incomplete, and thus hasn't been officially announced yet. If you want to view a complete list of read and unread topics that were posted within a certain timespan, the 'View most recent posts' link on the index page works great for that.