'std' doesn't work in settings page

When the value is set, it displays properly. But when I delete the value, std value doesn't work as expected. Please help.

/**
 * Register Settings Pages
 */
add_filter( 'mb_settings_pages', 'prefix_options_pages' );
function prefix_options_pages( $settings_pages ) {
	$settings_pages[] = array(
		'id'            => 'my-options',
		'option_name'   => 'my_options',
		'menu_title'    => __( 'Theme Options', 'textdomain' ),
		'submenu_title' => __( 'General', 'textdomain' ),
	);
}

/**
 * Register Fields
 */
add_filter( 'rwmb_meta_boxes', 'prefix_options_meta_boxes' );
function prefix_options_meta_boxes( $meta_boxes ) {
	$meta_boxes[] = array(
		'id'             => 'general', // Metabox ID
		'title'          => __( 'General', 'textdomain' ), // Metabox Title
		'settings_pages' => 'my-options', // Settings Page ID
		'fields'         => array(
			array(
				'id'    => 'test', // Field ID
				'name'  => __( 'Test', 'liberstart' ), // Field Name
				'type'  => 'text', // Field Type
				'clone' => false,
				'std'   => array( 'std test' ),
			),
		),
	);
}