ALTER from DECIMAL to BIGINT UNSIGNED returns a wrong result

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a DECIMAL(30,0));

INSERT INTO t1 VALUES (CAST(0xFFFFFFFFFFFFFFFF AS UNSIGNED));

SELECT * FROM t1;

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

| a                    |

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

| 18446744073709551615 |

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

ALTER TABLE t1 MODIFY a BIGINT UNSIGNED;

SELECT * FROM t1;

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

| a                   |

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

| 9223372036854775807 |

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