INET6 cannot be converted to BINARY(16) (requires clarification in documentation)

CREATE OR REPLACE TABLE t1 (a INET6);

INSERT INTO t1 VALUES ('2001:db8::ff00:42:8329');

ALTER TABLE t1 MODIFY a BINARY(16);

select cast(a as binary(16)) from t1;

show warnings;

MariaDB [test]> ALTER TABLE t1 MODIFY a BINARY(16);

ERROR 1406 (22001): Data too long for column 'a' at row 1

MariaDB [test]> select cast(a as binary(16)) from t1;

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

| cast(a as binary(16)) |

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

| 2001:db8::ff00:4      |

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

1 row in set, 1 warning (0.000 sec)

MariaDB [test]> show warnings;

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

| Level   | Code | Message                                                        |

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

| Warning | 1292 | Truncated incorrect BINARY(16) value: '2001:db8::ff00:42:8329' |

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

1 row in set (0.000 sec)