Bookmarking "first post" bookmarks last post

At the bottom of a topic there's the Bookmark button with tooltip "Click to bookmark the first post on this topic", but when I use it and subsequently use the resulting bookmark to return to the thread it takes me to the last post. On the page that lists all my bookmarks this bookmark also previews the text of the last post of the thread and not the first.

Though after using that button, neither the first nor last post is actually marked as bookmarked in the thread, making it a bit difficult to confirm whether the button actually does what it says.