This snippet for the Learndash add-on prevents the registration button URL on the child course from being overwritten.
add_action( 'broadcast_learndash_merge_existing_course_meta', 'bc_broadcast_learndash_merge_existing_course_meta' );
function bc_broadcast_learndash_merge_existing_course_meta( $action )
{
$action->keys []= 'sfwd-courses_custom_button_url';
}