Featured image now replacing header image in posts.

When I select a featured image, it now REPLACES the header image when I click on the post. This happens no matter what size the featured image is. I want to see the featured image in the post summary on the home page, and see the image itself in the blog post - but I do not want it to replace the header image once you click on the post link. This seems to be a change. Previously the featured image did not do this. Thanks for any help.