That's probably because the board thinks you've read all messages into that specific topic.
If there are unread topics, you'll see it like this, with a little icon that lets you move to the first unread post:
Clipboard Image.jpg
Is instead you've read all posts inside a topic, you won't see that icon and clicking on the title will bring you to the first post:
Clipboard Image (1).jpg
To always go to the latest post, click on the symbol on the right of the last post author:
Clipboard Image (2).jpg
It was always like this, but perhaps the board "lost" track of the posts you've seen already.
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