Lateral derived optimization causes incorrect results with <=>

CREATE DATABASE testdb;

USE testdb;

CREATE TABLE test1 (

    num1 INT NULL,

    KEY (num1)

);

INSERT INTO test1 VALUES (NULL), (1);

CREATE TABLE test2 (

    num2 INT NULL,

    KEY (num2)

);

INSERT INTO test2 VALUES (NULL), (2);

-- Set split_materialized to off to receive correct results

SET optimizer_switch='split_materialized=on';

WITH test AS (

    SELECT num1

    FROM test1

    GROUP BY num1

)

SELECT test.num1, test2.num2

FROM test2

-- Both test and test2 have a NULL value, so this join should return one row.

INNER JOIN test ON test.num1 <=> test2.num2

-- test2.num2 is NULL for the only expected row, but this condition is here to

-- make the optimizer use lateral derived optimization. Removing this condition

-- causes the query to return correct results.

WHERE test2.num2 IS NULL;

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

| num1 | num2 |

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

| NULL | NULL |

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

1 row in set

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

| id   | select_type     | table      | type | possible_keys | key  | key_len | ref               | rows | Extra                    |

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

|    1 | PRIMARY         | test2      | ref  | num2          | num2 | 5       | const             | 1    | Using where; Using index |

|    1 | PRIMARY         | <derived2> | ref  | key0          | key0 | 5       | testdb.test2.num2 | 1    | Using where              |

|    2 | LATERAL DERIVED | test1      | ref  | num1          | num1 | 5       | testdb.test2.num2 | 1    | Using index              |

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