Morning... We've just released an improvement/fix to the reply functionality.
Until recently, it wasn't impossible to directly reply to the last post in a thread as the last post box had been used instead to reply to the thread as a whole. Some further work has been done to remedy this.
Now, the reply box that's displayed at the bottom of the page will reply to the thread as a whole (ie. effectively a reply to the original post) and each individual post, including the last one, has its own 'Reply' button which opens a new reply box.
To ensure that it's clear which box is which, the main reply button is now marked as 'Reply to thread'.
I hope this is helpful