MEDIUMINT returns a wrong length on conversion to string

CREATE OR REPLACE TABLE t1 (a MEDIUMINT, b MEDIUMINT UNSIGNED);

CREATE OR REPLACE TABLE t2 AS SELECT

  CONCAT(a),

  CONCAT(b),

  COALESCE(a,''),

  COALESCE(b,'')

FROM t1;

DESC t2;

+----------------+------------+------+-----+---------+-------+

| Field          | Type       | Null | Key | Default | Extra |

+----------------+------------+------+-----+---------+-------+

| CONCAT(a)      | varchar(9) | YES  |     | NULL    |       |

| CONCAT(b)      | varchar(8) | YES  |     | NULL    |       |

| COALESCE(a,'') | varchar(9) | YES  |     | NULL    |       |

| COALESCE(b,'') | varchar(8) | YES  |     | NULL    |       |

+----------------+------------+------+-----+---------+-------+