How do I overwrite the id of a form button?

I am using a parameter attr

to set a custom id for a delete form button:

{{ form_widget(form.delete, { 'label': 'myCustomLabel', 'attr': {'id': 'myCustomId'} }) }}

      

        
        
        
      

    

But it works with anything ( 'class' ) other than the 'id' attribute . The id is still 'form_delete' and I cannot change it even with the form builder:

$this->createFormBuilder(null, ['csrf_protection' => false])
        ->setAction($this->generateUrl('task_delete', array(
            'prefix' => self::getTaskMapper()::getPrefix($task),
            'id' => $task->getId()
        )))->add('delete', SubmitType::class, [
            'label' => 'delete',
            'attr' => ['id' => 'MyCustomId']
        ])
        ->setMethod('DELETE')
        ->getForm();

      

        
        
        
      

    

Why is this so? How to rewrite it?

Rewriting the form id works well.