Ya, missing something very important
add_filter('acf/save_post', 'post_password_from_acf', 20);
function post_password_from_acf($post_id) {
if (get_post_type($post_id) != 'your-post-type') {
return;
}
$password = get_field('your-acf-field', $post_id);
$post = get_post($post_id);
$post->post_password = $password;
remove_filter('acf/save_post', 'post_password_from_acf', 20);
wp_update_post($post);
add_filter('acf/save_post', 'post_password_from_acf', 20);
}