Možná zkuste přesunout s_series as s
...
public function getEpisodes($limit = 12, $order = 'id_e', $das = 'desc')
{
$this->db->select('e.id_e, e.id_s, e.num_e, s.id_s, s.nom_s, s.thumb_s')
// Move s_series as s
->from('s_episodes as e')
->join('s_series as s', 'e.id_s = s.id_s', 'left')
->order_by('e.'.$order, $das)
->limit($limit);
$query = $this->db->get();
return $query->result_array();
}