Wrong result with split materialized query

CREATE TABLE t1 (

n1 int NOT NULL, n2 int NOT NULL, c1 char(1) NOT NULL,

KEY c1 (c1), KEY n1_c1_n2 (n1,c1,n2), KEY n2k (n2)

) ENGINE=InnoDB CHARSET=latin1;

INSERT INTO t1 VALUES (0,2,'a'), (1,3,'a');

INSERT INTO t1 SELECT seq+1, seq+2, 'c' FROM seq_1_to_1000;

CREATE TABLE t0 (a int, b int) ENGINE=InnoDB;

INSERT INTO t0 VALUES (7,99),(1,3),(0,2);

ANALYZE TABLE t0,t1;

SELECT * FROM t0 WHERE t0.b IN

(SELECT t.n2 FROM t1, (SELECT n1, n2 FROM t1 GROUP BY n1, n2) t

WHERE t.n1 = t1.n1 AND t1.c1 = 'a');

a	b

1	3

0	2

SELECT * FROM t0 WHERE t0.b IN

(SELECT t.n2 FROM t1, (SELECT n1, n2 FROM t1 GROUP BY n1, n2) t

WHERE t.n1 = t1.n1 AND t1.c1 = 'a' UNION select -1);

Empty set (0.015 sec)