I find it erratic. Sometimes, posts from the night before still show up as unread the next day, yet, as you note, other times the posts from just a couple of hours ago change to being marked as read during the day, when I am right in the middle of catching up.
Originally Posted by Coon
I have gotten into the habit of checking the time on the last unread post before I start reading, so I know how far to go back in case they all change to 'read.'
“If you want to gather honey, don't kick over the beehive.” - Dale Carnegie