For me new posts stay in the new posts section until i leave the forum for a period of 45minutes.

IF u use the autolog on feature, when you navigate away from the forum, close firefox or whatever, itll keep you logged into the forum for 45mins, so if you come back the new posts will remain there, but if u dont come back, after the 45mins the new posts are old posts, and it tracks the new threads frm when u left!