A subquery with a union for DECIMAL and BIGINT returns zeros

DROP TABLE IF EXISTS t1;

CREATE TABLE t1 (a DECIMAL(17,1), b BIGINT) ENGINE=ColumnStore;

INSERT INTO t1 VALUES (1, 1);

INSERT INTO t1 VALUES (9999999999999999, 99999999999999999);

SELECT * FROM (SELECT a FROM t1 UNION SELECT b FROM t1) tu;

+------+

| a    |

+------+

|  0.0 |

|  0.0 |

|  0.0 |

+------+

SELECT * FROM (SELECT a FROM t1 UNION ALL SELECT b FROM t1) tu;

+------+

| a    |

+------+

|  0.0 |

|  0.0 |

|  0.0 |

|  0.0 |

+------+

SELECT a FROM t1 UNION SELECT b FROM t1;

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

| a                   |

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

|                 1.0 |

|  9999999999999999.0 |

| 99999999999999999.0 |

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

SELECT a FROM t1 UNION ALL SELECT b FROM t1;

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

| a                   |

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

|                 1.0 |

|  9999999999999999.0 |

|                 1.0 |

| 99999999999999999.0 |

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