I have noticed that the “new” count now includes new replies and not just new posts.
I personally was preferring the previous setting, so I could see if there were new threads (I can always go to “latest” to get both threads and overall posts.)
I’ve lost track of this change. It was first tried in 2023, and reverted, but now it’s on again and on by default for everyone. Not sure if/how it’s different, but I didn’t like it back then (when I was paying closer attention to discourse dev).
I suppose we may be able to disable it.