Custom Webform Element won't get rendered

I've tried to implement a custom Webform element, just a simple checkbox. The element is available in the Webform UI, I can add it to my forms, but it never gets rendered. I have no idea what I'm missing. This is my custom plugin code:

<?php

namespace Drupal\mymodule\Plugin\WebformElement;

use Drupal\webform\Plugin\WebformElement\Checkbox;

/**
 * Provides a custom 'checkbox' element.
 *
 * @WebformElement(
 *   id = "my_checkbox",
 *   label = @Translation("Custom checkbox"),
 *   description = @Translation("Provides a custom checkbox form element."),
 *   category = @Translation("Custom elements"),
 * )
 */
class MyCheckbox extends Checkbox {

}

So I'm only extending the existing Webform Plugin 'Checkbox', changing the annotations.

This goes to modules/custom/mymodule/src/Plugin/WebformElement/MyCheckbox.php

But the element isn't rendered in any form I'm adding it to.

I'm using the latest beta version of Webform, beta13. Thanks for any hints!

Regards,
Boris