The plaintxtBlog theme overwrites the WP default gallery shortcode handler. I don’t know of many themes that do this. I had to comment out the code in the functions.php file in order to let the default WP shortcode handler do it’s job. Which is fine for me, but I think the author of this theme has used their own gallery shortcode handler to render theme specific attributes in the HTML. So, I thought it might be best if they updated their code to handle the newer gallery shortcode attributes, like include and exclude, etc.