I go to a thread I haven't read yet.
The default is that OP's post is at the top, followed by all the responses in order - as you would expect.
I click on Sort because I want to read the latest post first and now I have to choose 'oldest first' or 'newest first'.
It used to be that Sort would just reverse whatever order you were already on.
If I've just clicked on a new thread, the default is oldest -> newest, so the only other option is newest -> oldest.
The sort option should just reverse whatever you're on when you press it. The up/down arrow means that. It's just another pointless click.