Is instant_articles_should_submit_post really working?

There are some posts on our website that use features not supported by Instant Articles, like sliders from Nextend SmartSlider3 plugin.

Since they’re a relevant part of the post, we chose to block them from being submitted to IA, using the instant_articles_should_submit_post:

add_filter( 'instant_articles_should_submit_post', 'custom_instant_articles_should_submit_post', 10, 2 );

function custom_instant_articles_should_submit_post($should_show, $IA_object) {
  if ($should_show) {
    $post = get_post($IA_object->get_the_id());
    if (strpos($post->post_content, '[smartslider') !== FALSE) {
      $should_show = FALSE;
    }    
  }
  return $should_show;
}

Not a fancy solution, but it should be working, as the following message appears on the post edit screen:

This post will not be available as Instant Article due to a rule created in your site.

However, the post was published to IA, regardless of the message.

Am I missing something?

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