Why affected_rows always returns -1?

I seem to have problem getting affected_rows when I INSERT and SELECT, it just returns -1 for some reason? I'm using a database class which I use all the time for my projects which uses MYSQLI prepare statements to avoid SQL injections.

Does anyone know why it returns -1 all the time? From what I have read it should be able to return affected rows on both INSERT and SELECT.

Database class

class database {
    protected $_mysqli;
    protected $_debug;
 
    public function __construct($host, $username, $password, $database, $debug) {
        $this->_mysqli = new mysqli($host, $username, $password, $database);
        $this->_debug = (bool) $debug;
        if (mysqli_connect_errno()) {
            if ($this->_debug) {
                echo mysqli_connect_error();
                debug_print_backtrace();
            }
            return false;
        }
        return true;
    }
 
    public function q($query) {
        if ($query = $this->_mysqli->prepare($query)) {
            if (func_num_args() > 1) {
                $x = func_get_args();
                $args = array_merge(array(func_get_arg(1)),
                    array_slice($x, 2));
                $args_ref = array();
                foreach($args as $k => &$arg) {
                    $args_ref[$k] = &$arg; 
                }
                call_user_func_array(array($query, 'bind_param'), $args_ref);
            }
            $query->execute();
            
            if ($query->errno) {
              if ($this->_debug) {
                echo mysqli_error($this->_mysqli);
                debug_print_backtrace();
              }
              return false;
            }
 
            if ($query->affected_rows > -1) {
                return $query->affected_rows;
            }
            $params = array();
            $meta = $query->result_metadata();
            while ($field = $meta->fetch_field())