Wrong Result: Inconsistent behavior between Index Lookup and Table Scan

CREATE TABLE t1(

  c1 BIT, 

  c2 BIT NOT NULL, 

  UNIQUE (c2, c1)

);

INSERT INTO t1(c1,c2) VALUES (b'0',b'1'), (b'1',b'1');

INSERT INTO t1(c2) VALUES (b'0');

SELECT c2 FROM t1 

WHERE (INET_NTOA('2016-12-24 09:00:00') NOT IN (SELECT c2 FROM t1));

+--------+

| c2     |

+--------+

| 0x00   |

| 0x01   |

| 0x01   |

+--------+

3 rows in set, 6 warnings

SELECT c2 FROM t1 IGNORE INDEX (c2) 

WHERE (INET_NTOA('2016-12-24 09:00:00') NOT IN (SELECT c2 FROM t1 IGNORE INDEX (c2)));

Empty set, 6 warnings