Alright…It appears that setting the type of topics, as in; Normal, Sticky or Super Sticky, changes the behavior of the shortcode [bbp-single-forum id=].
I made all test topics’ type in both forums, to Normal and the problem disappeared. Same behavior with Sticky.
To wrap-up, a type Super-Sticky, will get these forums show up, no matter that you specify an exact one with the [bbp-single-forum id=] shortcode.
