Thank you for the details! I’ve found the problem 🙂
It was the configuration. The Relevance Options -> Categories Taxonomies -> Should all terms match exactly? option was enabled. It’s a very strict option and is mostly used for site with thousands of posts for better relevance. I’ve turned it off, and the related content started appearing 🙂
You can remove the shortcode if you want from the content because the Show the plugin under content? is activated on the Layout Options -> Output placement panel, so the plugin is automatically displayed under the content.