Why this query gives always the same result and not random?

I've this query, and everytime I run it it gives always the same result. But I'm ordering only by location_id a random result. So it should still keep random but just order by location_id. Right?

$sql = 'SELECT location_id, ad_id, ad_code
            FROM (
                SELECT al.location_id, a.ad_id, a.ad_code
                FROM ' . $this->ad_locations_table . ' al
                LEFT JOIN ' . $this->ads_table . ' a
                    ON (al.ad_id = a.ad_id)
                WHERE a.ad_enabled = 1
                    AND (a.ad_end_date = 0
                        OR a.ad_end_date > ' . time() . ")
                    $sql_where_views
                    $sql_where_clicks
                    AND " . $this->db->sql_in_set('al.location_id', $ad_locations) . '
                ORDER BY (' . $this->sql_random() . ' * a.ad_priority) DESC
            ) z
            ORDER BY z.location_id';

And I've saw that if I remove the ORDER BY z.location_id' it will work great and it will random. So the order by seems to be ordering not only by location_id but ad_id too -.-

I've made some changes to the query above to simplify:

$sql = 'SELECT al.location_id, a.ad_id, a.ad_code
                FROM ' . $this->ad_locations_table . ' al
                LEFT JOIN ' . $this->ads_table . ' a
                    ON (al.ad_id = a.ad_id)
                WHERE a.ad_enabled = 1
                    AND (a.ad_end_date = 0
                        OR a.ad_end_date > ' . time() . ")
                    $sql_where_views
                    $sql_where_clicks
                    AND " . $this->db->sql_in_set('al.location_id', $ad_locations) . '
                GROUP BY al.location_id, a.ad_id, (RAND() * a.ad_priority) DESC';

It gives the exact same result, and MySQL 5.7+ asks for a.ad_id on the groupby orelse it gives error "sql_mode=only_full_group_by".

So right now I'm using this code, similar to the above, just change the last line GROUP BY to ORDER BY:

$sql = 'SELECT al.location_id, a.ad_id, a.ad_code
                FROM ' . $this->ad_locations_table . ' al
                LEFT JOIN ' . $this->ads_table . ' a
                    ON (al.ad_id = a.ad_id)
                WHERE a.ad_enabled = 1
                    AND (a.ad_end_date = 0
                        OR a.ad_end_date > ' . time() . ")
                    $sql_where_views
                    $sql_where_clicks
                    AND " . $this->db->sql_in_set('al.location_id', $ad_locations) . '
                ORDER BY al.location_id, (' . $this->sql_random() . ' * a.ad_priority) DESC';

And now the query works and gives random results. I still doesn't understand why the 1st query doesn't work... Do anyone knows why?

Thanks in advance!