hook output not working

With the order complete email customized and enabled. I added the following snippet to output a simple link before & after order table. While the hooks are called I don’t see the link being outputted in my email.

If I disable the customized email, the link is displayed correctly in email.

add_action( 'woocommerce_email_before_order_table', 'output_extra_link', 10, 4);
add_action( 'woocommerce_email_after_order_table', 'output_extra_link', 10, 4);

function output_extra_link($order, $sent_to_admin, $plain_text, $email) {
  echo <<<LINK
<a href="/foo">bar</a>
LINK;
}

Bug in plugin? How can I get the link to show?

Thanks
Shawn