It works inverting the terms 🙂
add_action('acf/save_post', 'move_image_to_post_thumbnail');
function move_image_to_post_thumbnail($post_id) {
if (get_field('_thumbnail_id', $post_id, false)) {
// get unformatted value
// we did it above too, no point formatting the value for this
$image_id = get_field('_thumbnail_id', $post_id, false);
update_post_meta($post_id, 'image', $image_id);
}
}