It's just a small matter of usability. The function works and I use it every time I come here.
From my point of view it looks like, when I click the mark as read and the read flag is set, there is then another call to my unread replies query, which naturally produces no results, so I am presented with a search results page that is effectively blank. There is no content on that page other than a few links in the forum header, which I will most often click the unread posts link to read any new posts or replies to other forum members posts (I do not know what results if clicking mark as read from this page, however, but the forum index would seem a logical landing page).
Maybe the algorithm cannot be modified or is not worth the effort, but just thought I would ask.