I also noticed that behavior on both this and the old version. I think Phil is right that it has something to do with the way pages are cached, although for me there doesn't seem to be a difference between mobile and desktop. I'm not complaining mind you. I don't think it's a problem, just an interesting behavior. And to Brian's point, I like how easy the new site makes it to clear the Unread filter and see threads I've read recently. Makes it much easier to re-open a thread if you have a new idea on how to answer it.