Before, unread posts in a topic you were tracking were split into two groups: A grey bubble for unread posts that were made before the last time you visited the topic; and a blue bubble for unread posts that were made after the last time you visited the topic. (I’m using the built in dark theme.) This has stopped working recently, and now everything shows up in the blue bubble. I think it worked this way at least on the Latest and Unread tabs. I don’t remember if it showed up elsewhere.
This should be pretty easy to reproduce. Just find a topic with a lot of posts. Read through a handful of posts and then mark the topic as tracking. Then go to Latest or Unread and you’ll see the blue bubble instead of the expected grey bubble.