JS and CSS not loaded if site base url is not a regular one

In an environment like the one suggested above, I could get the module working only if the css and js libraries were attached explicitely. So I had to modify the term_reference_tree.module file by adding the following code:

function term_reference_tree_page_attachments(array &$attachments) {
  $attachments['#attached']['library'][] = 'term_reference_tree/term_reference_tree_js';
  $attachments['#attached']['library'][] = 'term_reference_tree/term_reference_tree_css';
}