update WooCommerce product attributes on save_post_product not working

I want to develop a function, that update the attribute sizes-eu of my WooCommerce products. This should happen every time I save/update the product.

So basically I use the save_post_product action with a custom function as followed:

add_action('save_post_product', 'update_product_attribute_sizes' );
function update_product_attribute_sizes ( $post_id )  {

    $sizes = array( '41', '42', '43', '44' );
    
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return $post_id;
    }

    if ( ! current_user_can( 'edit_product', $post_id ) ) {
        return $post_id;
    }
    
    wp_set_object_terms( $post_id, $sizes, 'pa_size-eu' );

}

An alternative solution I tried was this:

add_action('save_post_product', 'update_product_attribute_sizes' );
function update_product_attribute_sizes ( $post_id )  {

    $sizes = array(
        'name' => 'pa_size-eu',
        'value' => '41 | 42 | 43 | 44',
        'is_visible' => '1',
        'is_variation' => '0',
        'is_taxonomy' => '1'
    );
    
    if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) {
        return $post_id;
    }

    if ( ! current_user_can( 'edit_product', $post_id ) ) {
        return $post_id;
    }
    
    update_post_meta($post_id, '_product_attributes', $sizes);
}

These are my attributes:

and here is the taxonomy:

Unfortunately my attributes are not updated when saving the product and I don’t know why. Can anyone here give me some advice what the error might be?