If no page header content is set, it will show the featured image.
The featured image will only be replaced when there’s some sort of content set in the page header metabox.
The Page Header metabox is basically a more advanced featured image field, which is why it overwrites the featured image functionality. Making it so they both worked hand in hand would require a decent re-write of the code, and would probably make a lot of people mad at me when they updated π
Are you wanting to display both the featured image and the page header content at the same time? What would you want it to look like?