MYISAM: Return wrong value with group by

CREATE DATABASE IF NOT EXISTS test_db;

CREATE TABLE IF NOT EXISTS test_db.test (

    id                				INT,

	flag			                BIT,

    parent_id						INT,

    PRIMARY KEY (id)

)  ENGINE=MYISAM;

INSERT INTO test_db.test

(id, flag, parent_id)

VALUE

(1, 1, NULL),

(2, 0, 1),

(3, 0, NULL)

ON DUPLICATE KEY UPDATE

	flag = VALUES(flag);

SELECT

    pt.id,

    pt.flag,

    GROUP_CONCAT(ct.id) child_ids

FROM test_db.test pt

LEFT JOIN test_db.test ct ON ct.parent_id = pt.id

WHERE

	pt.parent_id IS NULL

GROUP BY

	pt.id