Yoast not excluding cpt from sitemap

Hi,

We have created a custom post type and for some reasons it has to be set to public. Yet we dont want it to be indexed or showed in the sitemap.

I have used the filter provided on the website:

function sitemap_exclude_post_type( $excluded, $post_type ) {
    return $post_type === 'my-custom-post-type';
}

add_filter( 'wpseo_sitemap_exclude_post_type', 'sitemap_exclude_post_type', 10, 2 );

Somehow, the custom post type is still displayed in the sitemap

What am i missing here?

Kind regards, Kees