LP:883976 - exists with DISTINCT & ORDER BY returns incorrect result

CREATE TABLE t1(pk INT PRIMARY KEY, a INT, INDEX idx(a));
INSERT INTO t1 VALUES (1, 10), (3, 30), (2, 20);
CREATE TABLE t2(pk INT PRIMARY KEY, a INT, b INT, INDEX idxa(a));
INSERT INTO t2 VALUES (2, 20, 700), (1, 10, 200), (4, 10, 100);
SELECT * FROM t1
WHERE EXISTS (SELECT DISTINCT a FROM t2 WHERE t1.a < t2.a ORDER BY b);

returns 3 rows but should return only one. Without DISTINCT or ORDER BY result is correct.

repeatable in 5.1 (actually incorrect result present in test suite results)