How to keep the plugin submenu open on viewing a custom version of users.php?

You must use a different parent slug (first parameter) to keep the user menu as you want.

An example,

add_action('admin_menu', 'register_subscriber_submenu_page');

function wpdocs_register_subscriber_submenu_page() {
      add_submenu_page(
           'users.php',
           'Subscribers',
           'Subscribers',
           'manage_options',
           'custom-submenu-page', 
           'custom_callback_function'
      );
  }

Try to prefix your plugin page slug, you can have conflict with such a name : ‘subscribers’ -> your-plugin-subscribers .

You will find more details add_submenu_page

Hope it helps