The actual bug is in advanced-custom-fields/core/fields/relationship.php in line 496:
At the moment it is: $post_type_object = get_post_type_object( get_post_type( ) );
And it should actually be: $post_type_object = get_post_type_object( get_post_type( $p->ID ) );
Otherwise it is getting the post type of the current post, intead of the selected post for the relationship block…