A few times I have gone into the forum and there has been a new post, but the “New Posts” or “Unread”, (can’t remember what it was), is missing.
Normally I just keep hitting that button if there are new posts.
I find it easy to use the “Latest” panel on the right side to find new or unread topics.
I updated Discourse a two or three weeks ago and otherwise haven’t changed anything with it recently. I also saw “Unread (2)” on the main page so maybe it was just a temporary glitch?