AS pointed out by review by Sensio, current code will lead to new cache generated every time the content returned from legacy is different.
The code causing the issue:

# eZ\Publish\Core\MVC\Legacy\View\TwigContentViewLayoutDecorator
$twigContentTemplate = <<<EOT
{% extends "{$layout}" %}
{% block {$options['contentBlockName']} %} {$contentViewClosure( $params )}
{% endblock %}
EOT;

Two solutions are available:

• Having a physical template and use it directly.
• Changing the template above to display the content as a twig variable:

  $twigContentTemplate = <<<EOT
  {% extends "{$layout}" %}
  {% block {$options['contentBlockName']} %}
  {{ content|raw }}
  {% endblock %}
  EOT;