Have reviewed your install. Your issue is because you're not protecting the articles. You're protecting their menu items. This is a very different level of protection. Protecting menu items does exactly that and only that it prevents direct access to that menu item.
Within your plans you need to use the Category or Article protection under Integrations > Content > For read access. I've confirmed this works on your site by protecting the article itself mentioned in your private message where you'll see it works as expected now with both image and intro protected.
I recommend protecting categories instead of individual articles or menu items though as will be substantially easier to maintain going forward. I've added a feature request to protect content by Tag as well. I might be able to have that implemented next week as that might be a better fit for you.
forge.joomlapolis.com/issues/9706
Regardless you'll need to go through your articles and replace that menu based protection with article or category based.