When I update an add-on, the description is automatically added to the Discussion of the Resource. Then I click New Posts and there is the update description showing as anew post. It's not exactly new to me as I just wrote it.
There is code that marks the thread as read again only if you had previously read it. This may have explicitly been the case, but there's also another situation where it was the case at the time, but there was a post that was deleted between you updating the resource and reading the thread (leading to you only seeing your post as new).