使用针对更新元的save_post操作（自定义帖子类型）操作自定义字段（在acf中创建）

Question

in custom post type product of woocommerce, I have a some custom fields (created in ACF) and I need to take action when I save or update the post, that is, I want to add the values ​​of the fields in woocommerce attributes. This is my example code. I have a text field called : ISBN , and I try to pass the value in meta attribute:

//save_post_NAME_CPT
add_action('save_post_product', 'update_post_meta_subito' , 10, 3 ); 

function update_post_meta_subito( $post_id, $post, $update ) { 

        //get field isbn of current post
        $prova_termine = get_field( "isbn", $post_id );

         //add value field in global pa_autore attribute
        $term_taxonomy_ids = wp_set_object_terms( $post_id, $prova_termine, 'pa_autore', true );

        //add value field in current attribute post
        $data = Array(

        'pa_autore'=>Array( 
        'name'=>'pa_autore', 
        'value'=>$prova_termine, 
        'is_visible' => '1', 
        'is_variation' => '1',
        'is_taxonomy' => '1'
        ),

        );

        update_post_meta( $post_id, '_product_attributes',$data);
    }

This not works! it does not pass any value! But in this case it works:

add_action('save_post_product', 'update_post_meta_subito' , 10, 3 ); 

function update_post_meta_subito( $post_id, $post, $update ) { 

        $prova_termine='ronaldo';

        $term_taxonomy_ids = wp_set_object_terms( $post_id, $prova_termine, 'pa_autore', true );

        $data = Array(

        'pa_autore'=>Array( 
        'name'=>'pa_autore', 
        'value'=>$prova_termine, 
        'is_visible' => '1', 
        'is_variation' => '1',
        'is_taxonomy' => '1'
        ),

        );

        update_post_meta( $post_id, '_product_attributes',$data);
    }

So, in the first function does not take the value of the field, where am I wrong?