Adding submenu to Metabox settings page replacing parent menu

I trying to add an Import and export function to my settings page. First, I have created settings page with Metabox plugin using following code

// Register myplugin settings page
add_filter( 'mb_settings_pages', 'myplugin_options_page' );
function myplugin_options_page( $settings_pages ) {
    $settings_pages[] = array(
        'id'          => 'myplugin_settings',
        'option_name' => 'myplugin_settings',
        'menu_title'  =>  'Plugin settings',
        'icon_url'    => 'dashicons-edit',
        'style'       => 'boxes',
        'columns'     => 1,
        'capability'  => 'edit_theme_options',
        'tabs'        => array(
          'settings_tab' => esc_html__('Settings', 'myplugin'),
          'help'     => esc_html__('Help', 'myplugin'),
        ),
        'position'    => 68,
    );
    $settings_pages[] = array(
        'id'          => 'myplugin-export',
        'option_name' => 'myplugin-export,
        'menu_title'  => esc_html__('Export settings', 'myplugin'),
        'parent'      => 'myplugin_settings',
        'style'       => 'boxes',
        'columns'     => 1,
        'capability'  => 'edit_theme_options',
    );
    return $settings_pages;
}

As metabox plugin doesn't have settings import feature I am trying to add a custom page for importing and tried to add a sub menu inside the Parent menu created with the Metabox plugin with the menu slug 'myplugin_settings'.

add_action('admin_menu', 'add_import_menu');
function add_import_menu() {
  add_submenu_page( 'myplugin_settings', 'Import settings', 'Import settings', 'edit_theme_options', 'new-submenu', 'plugin_submenu_page' );
}

This is replacing the parent menu "Plugin settings" ('myplugin_settings') with Import settings. Not sure what I am doing wrong.