I haven't read any posts over the past few days, but everything here is marked "read".
I had the same thing when I returned from my trip to the states, the 25th of last month, not having been at my regular computer where cookies are stored, but I had been at a different computer and used it to check the forge. All messages appeared 'read' at my regular computer (while there had been new ones in the intervening time).
After that, everything resumed acting normal. Possibly some time-out on the cookie or the size of the 'new posts' list?