Wrong results with LEFT JOIN and subqueries with UNION and GROUP BY

SET hive.auto.convert.sortmerge.join=true;

CREATE TABLE tbl (key int,value int);
INSERT INTO tbl VALUES (1, 2000);
INSERT INTO tbl VALUES (2, 2001);
INSERT INTO tbl VALUES (3, 2005);

SELECT sub1.key, sub2.key
FROM
  (SELECT a.key FROM tbl a GROUP BY a.key) sub1
LEFT OUTER JOIN (
  SELECT b.key FROM tbl b WHERE b.value = 2001 GROUP BY b.key
  UNION
  SELECT c.key FROM tbl c WHERE c.value = 2005 GROUP BY c.key) sub2 
ON sub1.key = sub2.key;