Function sanitize_title() does not appear to be working

As you can see in the source code of sanitize_title(), all the function does is 3 things

1. apply the function remove_accents(), supposedly to remove accents
2. apply the filter sanitize_title
3. return either $title or $fallback_title

Since you’re loading only some partial WordPress files, you’ll probably be missing all the goodies of the filter.

I dug a bit through the source code, and it looks like sanitize_title_with_dashes() is doing most of the work. The function resides within /wp-includes/formatting.php as well, so in your case I’d just use this one in conjunction with remove_accents() like so

$sanitized_string = sanitize_title_with_dashes( remove_accents( $unsanitized_string ) );