Just to clarify. For options page acf/save_post
works as follows:
function my_acf_save_post($post_id) {
var_dump($post_id); // returns: 'options'
var_dump($screen->base); // returns: '[parent-slug]_page_[your-menu-slug]'
exit();
}
add_action('acf/save_post', 'my_acf_save_post', 20);