CHARSET(column) returns different result with and without ROLLUP

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (d DATE) ENGINE=MyISAM;

INSERT INTO t1 VALUES ('2018-12-12');

SELECT d, CHARSET(d) AS f FROM t1 GROUP BY d;

SELECT d, CHARSET(d) AS f FROM t1 GROUP BY d WITH ROLLUP;

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

| d          | f      |

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

| 2018-12-12 | binary |

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

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

| d          | f      |

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

| 2018-12-12 | latin1 |

| NULL       | binary |

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