Question: how to insert heading during DocumentParsedEvent and have the table of contents pick it up?

[Ambient-Impact]

Hi there. I've gone through the documentation and issues numerous times but cannot seem to get this to work as expected. I'm trying to insert a Heading before a FootnoteContainer and have that heading be picked up correctly by a table of contents. I've managed to get the heading inserted and picked up by the heading permalink:

But then, the table of contents entry for it ends up with an empty item:

This is my listener attachment:

    $environment->addEventListener(
      DocumentParsedEvent::class,
      [\get_class($this), 'onDocumentParsed'],
      -10
    );

and the callbacks:

  public static function onDocumentParsed(DocumentParsedEvent $event) {
    /** @var \League\CommonMark\Block\Element\Document */
    $document = $event->getDocument();

    /** @var \League\CommonMark\Node\NodeWalker */
    $walker = $document->walker();

    while ($event = $walker->next()) {
      /** @var \League\CommonMark\Node\Node */
      $node = $event->getNode();

      if ($node instanceof FootnoteRef && $event->isEntering()) {
        static::alterFootnoteRef($document, $node);
      }

      if ($node instanceof FootnoteContainer && $event->isEntering()) {
        static::alterFootnoteContainer($document, $node);
      }
    }
  }

  protected static function alterFootnoteRef($document, $node): void {
    $existingReference = $node->getReference();

    $newReference = new Reference(
      $existingReference->getLabel(),
      $existingReference->getDestination(),
      '[' . $existingReference->getTitle() . ']'
    );

    $node->setReference($newReference);
    $document->getReferenceMap()->addReference($newReference);
  }

  protected static function alterFootnoteContainer($document, $node): void {
    /** @var \Drupal\Core\StringTranslation\TranslatableMarkup */
    $headingContent = new TranslatableMarkup('References');

    /** @var \League\CommonMark\Block\Element\Heading */
    $heading = new Heading(3, $headingContent);

    $heading->appendChild(new Text($headingContent));
    // $heading->addLine($headingContent);

    $node->insertBefore($heading);
  }

For some reason, the content I pass to the Heading doesn't seem to be rendered, nor using addLine() - and yes, I've tried a plain string instead of Drupal's TranslatableMarkup - but appending a new Text seems to work for that. The actual permalink slug is generated correctly too, so I can't really see what else I need to do.