it is obviously linked somewhere e.g. sitemaps not compatible with Seblod can create such problems. Also make sure that all your links on lists have menu target set explicitly, especially those in the module or list field.
This is a problem of how Joomla! works. You can see any article by passing the parameters to the url. You must ensure the right permissions when you create a content type, not only for the form, also for the content view or list&search view.
You must have in mind always that any kind of content stored as article will be visible with this kind of url: index.php?option=com_content&view=article&id=XXXX except those who has the right permissions.
If is the public access what is worrying you, just add a hidden field in the form called "Article Access" and go to button 2 Live / Live value and add the ID of the desired level access, 2 for Registed or 3 for Special, or any level access you have in your panel.