I also just managed to get it to work by removing the $_POST['return'] line. So I ended up with this:
$post_id = wp_insert_post($post);
do_action( 'acf/save_post' , $post_id ); // Save the fields to the post
wp_redirect( add_query_arg( 'updated', 'true', get_permalink( $post_id ) ) ); exit; // Redirect to the new post
you can also replace get_permalink( $post_id ) with a specific URL.
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