Some columns will be lost when a UDTF has multiple aliases in some cases

Only one column will be retained when putting UDTFs with multiple aliases and a top-level UNION together.

For example, the result of the following SQL should have three columns, c1, c2 and c3.

SELECT stack(1, 'a', 'b', 'c') AS (c1, c2, c3)
UNION ALL
SELECT stack(1, 'd', 'e', 'f') AS (c1, c2, c3);

However, It's only the c3 column which I can get.

+---------+
| _u1.c3  |
+---------+
| c       |
| f       |
+---------+