function my_acf_save_post( $post_id ) {
// get new value
$user_id = get_field('author');
if($user_id){
wp_update_post( array( 'ID'=>$post_id, 'post_author'=>$user_id) );
}
}
add_action('acf/save_post', 'my_acf_save_post', 20);
This is the code I am using
