Filter::process for ClassDiagram is broken

The result of process in

class FilterClassDiagram extends FilterBase {

...
  public function process($text, $langcode, $cache, $cache_id) {
...
    return $this->text;
  }

has

(string) <pre>
<object type="image/svg+xml" data="data:image/svg+xml;base64,PD94bWwg...</pre>

which somehow does not render anymore.

When previewing process is not touched which is weird.

When saving process is touched. But no result.

I tried Book and Article nodes with

<p>Hello</p>

<pre>
[classdiagram
\Drupal\Component\Plugin\PluginManagerInterface
]
</pre>

<p>there.</p>