https://www.advancedcustomfields.com/resources/acf-update_value/
add_filter('acf/update_value/name=custom_page_explainer', 'acf_save_description', 10, 3);
function acf_save_description($value, $post_id, $field) {
if (substr($post_id, 0, 5) != 'term_') {
return $value;
}
$term_id = intval(substr($post_id, 5));
//....
}