Here is the configuration of the frontend template, I used to do in v1:

ezpublish:
    system:
        default:
            pagelayout: "@ezdesign/pagelayout.html.twig"

            content_view:
                full:
                    article:
                        template: "@ezdesign/full/article.html.twig"
                        match:
                            Identifier\ContentType: [article]

The problem is when I view an Article in the backend (admin siteaccess), the frontend template is used.
default should not be used for admin interface.

Note, I tried to use site_group (frontend siteaccess group) instead of default but it's not working.
The only way to have the admin template and the frontend template is to use a siteacces (site instead of default). This is not great because you will have to define the same overrides for all the siteaccesses.

I think admin siteaccess should also use ez-design-engine. It will be good for developers if they want to override a template in the admin interface.