1) the validator shows that everything is fine
2) deactivate the page title
3) create a Header element
4) <h1 class = “entry-title” itemprop = “headline”> {{post_title}} </h1>
5) activate it for the whole site
6) the validator shows an error Warning: Article lacks heading. —- Consider using h2-h6 elements to add identifying headings to all articles.
If you use the Header element to display the <h1> post title then yes, it moves it outside of the article tag, this is a user design choice. If you want to maintain the H1 inside your content then you need to remove the H1 post title from the header element.
But the simple fix is to include better heading structure in your content which is why the validator suggests Consider using h2-h6 elements to add identifying headings to all articles.