I'm noticing the same thing here, but the time limit seems very short. I just opened a thread (for the first time) that was originally started on Sunday. I was automatically taken to the first unread post (posted today), which happened to be the 11th post in the thread. So it essentially thought I had read the first 10 posts from Sunday, when in fact, I hadn't.
So is there a time limit and if so, how long is it? Can it be increased to at least 7 days or something so that I'm not being taken to the middle of threads I haven't even started reading?