Issues with title-tag and document_title_parts

I ran your filter in my development area. It didn’t work. Then I switched off the Yoast SEO plugin, which I knew was also messing with the page title. Then it worked. So my suggestion would be another plugin is messing with it.

In the case of Yoast, it was a filter call to pre_get_document_title returning non empty. In that case wp_get_document_title is short circuited and the rest of the function, including the documents_title_parts filter, is not evaluated, as you can see from the first lines of code:

$title = apply_filters( 'pre_get_document_title', '' );
if ( ! empty( $title ) ) {
    return $title;
    }

So, I took your filter and changed the hook to pre_get_document_title. It didn’t work. Then I changed the priority to a higher level than the same filter in Yoast. Then it worked. So, I don’t know about your set-up, but I suggest you give this a try:

add_filter( 'pre_get_document_title', function( $title )
  {
    error_log('here');
    return $title;
  }, 999, 1 );