add_query_arg() for custom post type on all visible links

Filter post_type_link instead. This is reserved for custom post types, and you get the post object, so you don’t need a global variable for this.

is_admin() or add_filter( 'post_type_link', 'career_position_url', 10, 2 );

function career_position_url( $url, $post )
{
    global $career_mb;

    if ( 'career' !== $post->post_type )
        return $url;

    $career = $career_mb->the_meta();

    return add_query_arg( 'position', $career['career_position'], $url );
}