Wrong result for INSERT INTO t1 (datetime_field) VALUES (hybrid_function_of_TIME_data_type)

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a DATETIME);

INSERT INTO t1 VALUES (TIME'10:20:30'),(COALESCE(TIME'10:20:30'));

SHOW WARNINGS;

SELECT * FROM t1;

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

| Level   | Code | Message                                |

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

| Warning | 1265 | Data truncated for column 'a' at row 2 |

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

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

| a                   |

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

| 2016-11-22 10:20:30 |

| 0000-00-00 00:00:00 |

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

SET timestamp=UNIX_TIMESTAMP('2001-02-03 10:20:30.123456');

CREATE OR REPLACE TABLE t1 (a DECIMAL(30,6) DEFAULT COALESCE(CURRENT_TIMESTAMP(6)));

SHOW CREATE TABLE t1;

INSERT INTO t1 VALUES (CURRENT_TIMESTAMP(6));

INSERT INTO t1 VALUES (COALESCE(CURRENT_TIMESTAMP(6)));

INSERT INTO t1 VALUES();

SELECT * FROM t1;

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

| a                     |

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

| 20010203102030.123456 |

|           2001.000000 |

|           2001.000000 |

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