Because you are not in a loop that includes while (have_posts) { the_post()... WP is looking at the post type of the post in “The Loop”, the current page, and not the post referenced by$featured_post. This post type will not match.
You either need to use a loop that set the global $post as explained in the documentation for relationship fields https://www.advancedcustomfields.com/resources/relationship/
Or you need to use a different method for testing the post type, like this:
if ($featured_post->post_type == 'reizen') {
...
