The Gallery shortcode handler is part of WordPress core, and not generally something impacted by Themes.
Do you have the same problem when using a different Theme?
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.
Well, I’ll be!
Yeah, a current Theme would not be permitted to do that. This Theme hasn’t been updated for three years; you might want to consider using a more recent Theme, or simply keep using a forked version of it.
Geez! I didn’t notice how long it’s been since it was updated. I’ve been inheriting this theme for three years now…might be time to just roll my own!