'rest_value_cb' not working on ".../wp-json/wp/v2/posts" and ".../wp-json/wp/v2/posts/<post_id>"
so i changed this line from :
$values[ $cmb_id ][ $field->id( true ) ] = $field->get_data();
to:
$values[ $cmb_id ][ $field->id( true ) ] = $field->get_rest_value();