symfony: Can't create a child form type of the SubmitType

In my HTML I want to create a button which has HTML as a label (button text) because I want to use the material-icons. The problem is, that the SubmitType doesn’t transform the HTML to HTML but just prints it onto the button, so I wanted to create a custom form type which inherits from the SubmitType and renders the HTML. So I created a new FormType

<?php
namespace Acme\FrontendBundle\Form\Type;

use Symfony\Component\Form\AbstractType;
use Symfony\Component\Form\Extension\Core\Type\SubmitType;

/**
 * Class HtmlSubmitType
 *
 * This form type is used for submit buttons which need HTML for the data (e.g. label). The html rendering is defined
 * in the fields.html.twig file
 *
 * @category Form Type
 * @package  Acme\FrontendBundle\Form\Type
 * @link     http://www.example.com
 * @author   Theo Tzaferis <email@example.com>
 */
class HtmlSubmitType extends AbstractType
{
    /**
     * @inheritdoc
     */
    public function getParent()
    {
        return SubmitType::class;
    }
}

However, when using this form field in a FormType which has this as the default data_class 'data_class' => ResolvedVendor::class and adding it

->add(sprintf('quote_%s', $vendor->getToken()), HtmlSubmitType::class)

I’m always getting the error

Neither the property “quote_25v9nt1puvms” nor one of the methods “getQuote25v9nt1puvms()”, “quote25v9nt1puvms()”, “isQuote25v9nt1puvms()”, “hasQuote25v9nt1puvms()”, “__get()” exist and have public access in class “Acme\FrontendBundle\Model\Cart\ResolvedVendor”.

I don’t really understand it, since the documentation says

Here, the return value of the getParent function indicates that you’re extending the ChoiceType field. This means that, by default, you inherit all of the logic and rendering of that field type

and also it’s a submit button, why does it need to be mapped? Why does this error not get thrown when using the SubmitType but when extending from it? I also can’t set the option mapped to false, since it’s not a valid option for the SubmitType which makes sense, but then why am I getting the error?