Remove Parent Categories

I’m using a popular function to remove the parent category from the permalink:

add_filter( 'post_link', 'remove_parent_cats_from_link', 10, 3 );
function remove_parent_cats_from_link( $permalink, $post, $leavename )
{
    $cats = get_the_category( $post->ID );
    if ( $cats ) {
        // Make sure we use the same start cat as the permalink generator
        usort( $cats, '_usort_terms_by_ID' ); // order by ID
        $category = $cats[0]->slug;
        if ( $parent = $cats[0]->parent ) {
            // If there are parent categories, collect them and replace them in the link
            $parentcats = get_category_parents( $parent, false, '/', true );
            // str_replace() is not the best solution if you can have duplicates:
            // myexamplesite.com/luxemburg/luxemburg/ will be stripped down to myexamplesite.com/
            // But if you don't expect that, it should work
            $permalink = str_replace( $parentcats, '', $permalink );
        }
    }
    return $permalink;
}

Amp pages work fine for posts that do not have a parent category, however those that are tagged with a category that has a parent redirect to the original page url sans /amp/.

Any ideas on how I can fix this?

https://www.ads-software.com/plugins/amp/