Problem with add attributes via update_post_meta

Hello everybody,

an external wms sends the products to WooCommerce via API. The “brand” is passed on as a meta value.

I need “brand” as an attribute, so I wrote the following function in the functions.php:

add_action( 'save_post_product', 'get_meta_set_attribute' );
function get_meta_set_attribute( $id_prodotto ) {
	$tassonomia = "pa_marca";
	
	error_log(print_r($id_prodotto, true));
	$brand = get_post_meta($id_prodotto, 'meta_product_producer', true);
	error_log("Il brand è: " . $brand);
	
	$attributi_prodotto = get_post_meta( $id_prodotto, '_product_attributes', true);
	$posizione = count($attributi_prodotto);
	
	error_log(print_r($attributi_prodotto, true));
	
	$attributi_prodotto[$tassonomia] = array(
		'name' => $tassonomia,
		'value' => $brand,
		'position' => $posizione,
		'is_visible' => 1,
		'is_variation' => 0,
		'is_taxonomy' => 1
	);
	
	error_log(print_r($attributi_prodotto, true));
	
	try {
		$result = update_post_meta($id_prodotto, '_product_attributes', $attributi_prodotto); 
	} catch (Exception $e) {
      echo("<br />Errore: " . ($e->getMessage()));
  	}
	
	error_log("Il risultato della richiesta di aggiornamento è: " . $result);
}

Unfortunately there are two problems:

1. When I print the attributes already present in the product (size and color), it returns them to me without values … even if these exist within the product (see screens -> https://imgur.com/a/iQnGKue)

2. The update_post_meta function is executed successfully (I do not receive any errors), but the “brand” attribute (in Italian: marca) is not added to the product.

Can you help me to undestrood the problem? thanks