Incorrect column mapping in EXCEPT query involving UNION ALL with a false constant condition in a derived table

-- create table

DROP TABLE IF EXISTS t0;

CREATE TABLE t0 (

    c0 TEXT,

    c2 BIGINT UNSIGNED ZEROFILL

);

INSERT INTO t0 VALUES (NULL, 22105307), (NULL, 0);

-- query, expect: empty set, actual: {22105307, 0}

SELECT COALESCE(a, b) AS c0 FROM (

    SELECT t_sub0.c0 AS a, t_sub0.c2 AS b 

    FROM (SELECT * FROM t0 AS t_sub0 WHERE CAST('b' AS DECIMAL(65, 30))) AS t_sub0

    UNION ALL

    SELECT t_sub1.c0 AS a, t_sub1.c2 AS b 

    FROM (SELECT * FROM t0 AS t_sub1 WHERE NOT (CAST('b' AS DECIMAL(65, 30)))) AS t_sub1

) AS t_branch

EXCEPT

SELECT COALESCE(`t3`.`c0`, `t3`.`c2`) AS `c0` FROM `t0` AS `t3`;