As I posted before in a thread of the same title. The following still happens to me about once a week:
I'm reading through a thread of some number of posts, say a dozen. I'm part way through, and in the middle of reading an article. Then the timer goes off. Some, but not all, of the posts I've just read vanish from the Article Content pane. This, most irritatingly, includes the one I'm in the middle of reading. Whether new posts get added just then I don't know.
At the time this happens, I'm reading stuff, definitely not clicking "Hide read" buttons. An article I'm reading should not be considered "read" until it's no longer open.