I’m really not sure on this one. You may just have to avoid that one or use the external page as the workaround as you’re doing. I wish I had a better answer that could shed some light on it. When that data gets saved (for a textarea option for example), we’re just using common WP builtin sanitization functions to ensure that it can be safely saved to the database, and then saving all that data with update_post_meta.
Note: If you’re curious what the theme actually does to the text area option when prepping for saving it to the DB, see framework/admin/modules/options/options-sanitize.php and see the of_sanitize_textarea function.
As far as this forum, it’s running bbPress on top of WordPress. bbPress is the official forum plugin created by WordPress.