Wrong result of UNION .. ORDER BY GROUP_CONCAT()

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a INT) ENGINE=MyISAM;

INSERT t1 VALUES (1),(2),(3); 

SELECT 1 AS a UNION SELECT * FROM t1 GROUP BY a WITH ROLLUP ORDER BY GROUP_CONCAT(a ORDER BY 1);

+------+

| a    |

+------+

|    1 |

+------+

SELECT * FROM t1 GROUP BY a WITH ROLLUP;

+------+

| a    |

+------+

|    1 |

|    2 |

|    3 |

| NULL |

+------+

SELECT 1 AS a UNION SELECT * FROM t1 GROUP BY a WITH ROLLUP ORDER BY AVG(a);

SELECT 1 AS a UNION SELECT * FROM t1 GROUP BY a WITH ROLLUP ORDER BY SUM(a);