SP variables do not return expected NULL in summary rows in ROLLUP queries

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (id INT);

INSERT INTO t1 VALUES (1),(2);

DROP PROCEDURE IF EXISTS p1;

DELIMITER $$

CREATE PROCEDURE p1()

BEGIN

  DECLARE v INT DEFAULT 10;

  SELECT id, 10 AS n, COUNT(*) FROM t1 GROUP BY id, n WITH ROLLUP;

  SELECT id, v, COUNT(*) FROM t1 GROUP BY id, v WITH ROLLUP;

END;

$$

DELIMITER ;

CALL p1;

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

| id   | n    | COUNT(*) |

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

|    1 |   10 |        1 |

|    1 | NULL |        1 |

|    2 |   10 |        1 |

|    2 | NULL |        1 |

| NULL | NULL |        2 |

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

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

| id   | v    | COUNT(*) |

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

|    1 |   10 |        1 |

|    1 |   10 |        1 |

|    2 |   10 |        1 |

|    2 |   10 |        1 |

| NULL |   10 |        2 |

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