If there's a forum thread for a specific product, then when I click on "new" posts, I get sent to the top of the page instead of the new post, and there's a login popup, even if I'm already logged in. If the thread is elsewhere in the forums, i.e. not a specific product, I get either the new post, or the last post on the page before the new post.
Clicking on "new" should consistently go to the earliest new post in the thread, regardless whether it's a product thread or not, and regardless of where in the stream of posts that new post is.
I'm using Safari under Big Sur on an iMac.
Happens on Chrome on PC for me too, for as long as I can remember.