Wrong result with GROUP BY + multipart key + MIN/MAX loose scan

It looks like it is important to have range access, explain of incorrect execution:

+id select_type table type possible_keys key key_len ref rows filtered Extra
+1 PRIMARY t1 range NULL b 10 NULL 10 90.00 Using where; Using index for group-by
+2 SUBQUERY t2 ALL NULL NULL NULL NULL 2 100.00 Using where
+Warnings:
+Note 1003 select min(`test`.`t1`.`a`) AS `MIN(a)`,`test`.`t1`.`b` AS `b` from `test`.`t1` where (`test`.`t1`.`a` > (select `test`.`t2`.`a` from `test`.`t2` where (`test`.`t2`.`a` = 0))) group by `test`.`t1`.`b`

If reduce test suite table to INSERT INTO t1 VALUES (0,99),(9,99),(4,0),(7,0),(99,0),(7,0),(8,0),(99,0),(1,0); it works correctly with other explain:

+id select_type table type possible_keys key key_len ref rows filtered Extra
+1 PRIMARY t1 index NULL b 10 NULL 6 100.00 Using where; Using index
+2 SUBQUERY t2 ALL NULL NULL NULL NULL 2 100.00 Using where
+Warnings:
+Note 1003 select min(`test`.`t1`.`a`) AS `MIN(a)`,`test`.`t1`.`b` AS `b` from `test`.`t1` where (`test`.`t1`.`a` > (select `test`.`t2`.`a` from `test`.`t2` where (`test`.`t2`.`a` = 0))) group by `test`.`t1`.`b`

This is repeatable without a subquery:

DROP TABLE IF EXISTS t1;
CREATE TABLE t1 ( a int, b varchar(1), KEY (b,a));
INSERT INTO t1 VALUES (0,NULL),(9,NULL),(8,'c'),(4,'d'),(7,'d'),(NULL,'f'),(7,'f'),(8,'g'),(NULL,'j');

SELECT a , b FROM t1 WHERE a IS NULL OR b = 'z' ;
SELECT max(a) , b FROM t1 WHERE a IS NULL OR b = 'z' GROUP BY b;

Also, for a certain set of values, it is also repeatable in mysql 5.5 . So, it may be a legacy thing.

I checked the bug in 5.1 and it is repeatable.

Re-assigning to Timour because the problem is with min-max loose scan.

The following query also produces wrong result:

SELECT b, min(a) FROM t1 WHERE a = 7 OR b = 'z' GROUP BY b;
+------+--------+
| b | min(a) |
+------+--------+
| f | 7 |
+------+--------+

While the correct result is:

+------+--------+
| b | min(a) |
+------+--------+
| d | 7 |
| f | 7 |
+------+--------+