Filter not working

On the Yoast website, i see the following filter described:
https://developer.yoast.com/features/schema/pieces/product/
including an example:

add_filter( 'wpseo_schema_product', 'example_change_product' );
/** * Adds the color to the product schema. * 
* @param array $data The Schema Product data. * 
* @return array Schema.org Product data array. */

function example_change_product( $data ) {	
    $data['color'] = 'red';	
    return $data;
}

If i add this to a custom plugin, nothing happens in the schema output on a product page. I cleared the cache etc. The plugin is active. Am i something missing?