Using addElement to add support for unsupported elements does not appear to be consistent

[ebssalvail]

Hello, I am trying to add support for some additional elements (many of them HTML5) that are not supported by default. To do this, I followed the customize documentation here: http://htmlpurifier.org/docs/enduser-customize.html
However, I am seeing inconsistent results when trying to add elements in this way.

Below is an example that demonstrates my point.
I am using the latest version 4.18.0
My basic setup looks like this:

$config = HTMLPurifier_Config::createDefault();
$config->set('HTML.DefinitionID', 'html5-definitions');
$config->set('HTML.DefinitionRev', 1);
if ($def = $config->maybeGetRawHTMLDefinition()) {
    $def->addElement('figure', 'Block', 'Flow', 'Common');
    $def->addElement('figcaption', 'Inline', 'Flow', 'Common');
    $def->addElement('article', 'Block', 'Flow', 'Common');
}

$purifier = HTMLPurifier::instance($config);
$purifiedData = $purifier->purify($html);

The raw HTML I am testing with looks like:

<figure>test</figure>
<figcaption>test</figcaption>
<article>
    <h1>Test article heading</h1>
</article>

After running this HTML through the purifier, this is what I see being returned:

<figure>test</figure>
<figcaption>test</figcaption>
    <h1>Test article heading</h1>

As you can see, the figure and figcaption elements are not stripped as expected. However, article is still being removed.
How am I meant to add support for this? Is something in my config incorrect?
I have also tested with other elements like input, label, header, footer, section, etc and got the same results.

Is my only option trying to use another library with more HTML5 support like https://github.com/xemlock/htmlpurifier-html5 ?

EDIT:
After further testing, I have found that this config works and does not remove the article element:

$def = $config->getDefinition('HTML', true);
$def->addElement('figure', 'Block', 'Flow', 'Common');
$def->addElement('figcaption', 'Inline', 'Flow', 'Common');
$def->addElement('article', 'Block', 'Flow', 'Common');

However, I'm still confused on why it didn't work with my prior config as that is what is stated in the docs.