CompatFormTest: Debug why the `actual` result is empty when fieldset contains a checkbox?

[sukhwinder33445]

I noticed that the following test's actual result is empty when the fieldset contains a checkbox.

The form works fine in real usage, both via Form::assemble() and in inline forms (as in example below).

public function testMethodApplyDefaultElementDecorators(): void
{
    // A fieldset, a required checkbox, and a submit button should cover
    // all default element decorators.
    $form = new CompatForm();
    $form->applyDefaultElementDecorators();

    $fieldset = $form->createElement('fieldset', 'foo', [
        'label' => 'Fieldset Label',
        'description' => 'Fieldset Description',
        'id' => 'foo-id'
    ]);

    // Comment out the code below fixes the issue
    // vvvvvvvvvvvvvvvvv
    $fieldset->addElement('checkbox', 'fooAndBar', [
        'label' => 'Fieldset Label',
        'description' => 'Fieldset Description',
        'id' => 'fooAndBar-id'
    ]);
    $form->addElement($fieldset);

    // ^^^^^^^^^^^^^^^^^^^
    
    $form->addElement('submit', 'submit_form', ['label' => 'Submit Form']);

    $expected = <<<'HTML'
    <form class="icinga-form icinga-controls" method="POST">
      SOMETHING
    </form>
HTML;

    $this->assertHtml($expected, $form);
}