OK – I think I’ve solved it.
In my theme (Vellum) the [bbp-forum-index] shortcode entered on the new forum page seems to be deleted every time I add a forum. Adding the shortcode again rebuilds the forum list and displays them in the specified order. Weird, but at least it is a workaround.
Thank you, Robin W, for saving me from banging my head against a brick wall!
So, another possibility: is there a way of adding CSS to Forums, Topics and Replies (eg opening in Visual Composer)? I have the CSS to hide the one-page theme menu on a page, but don’t know how to add this to the bbPress entries.
Cheers!