LP:993459 - Execution of PS for a query with GROUP BY returns wrong result

The following sequence of commands returns a wrong result in MariaDb 5.2 (but not in 5.3/5.5) :

PREPARE s1 FROM "
SELECT c1, t2.c2, count(c3)
FROM ( SELECT 3 as c2 FROM dual WHERE @x = 1
UNION
SELECT 2 FROM dual WHERE @x = 1 OR @x = 2) AS t1,
( SELECT '2012-03-01 01:00:00' AS c1, 3 as c2, 1 as c3 FROM dual
UNION
SELECT '2012-03-01 02:00:00', 3, 2 FROM dual
UNION
SELECT '2012-03-01 01:00:00', 2, 1 FROM dual) AS t2
WHERE t2.c2 = t1.c2
GROUP BY c1, c2";

SET @x = 1;

SELECT c1, t2.c2, count(c3)
FROM ( SELECT 3 as c2 FROM dual WHERE @x = 1
UNION SELECT 2 FROM dual WHERE @x = 1 OR @x = 2 ) AS t1,
( SELECT '2012-03-01 01:00:00' AS c1, 3 as c2, 1 as c3 FROM dual
UNION
SELECT '2012-03-01 02:00:00', 3, 2 FROM dual
UNION
SELECT '2012-03-01 01:00:00', 2, 1 FROM dual ) AS t2
WHERE t2.c2 = t1.c2
GROUP BY c1, c2;
EXECUTE s1;
SET @x = 2;
SELECT c1, t2.c2, count(c3)
FROM ( SELECT 3 as c2 FROM dual WHERE @x = 1
UNION
SELECT 2 FROM dual WHERE @x = 1 OR @x = 2 ) AS t1,
( SELECT '2012-03-01 01:00:00' AS c1, 3 as c2, 1 as c3 FROM dual
UNION SELECT '2012-03-01 02:00:00', 3, 2 FROM dual
UNION SELECT '2012-03-01 01:00:00', 2, 1 FROM dual ) AS t2
WHERE t2.c2 = t1.c2
GROUP BY c1, c2;
EXECUTE s1;
SET @x = 1;
SELECT c1, t2.c2, count(c3)
FROM ( SELECT 3 as c2 FROM dual WHERE @x = 1
UNION SELECT 2 FROM dual WHERE @x = 1 OR @x = 2 ) AS t1,
( SELECT '2012-03-01 01:00:00' AS c1, 3 as c2, 1 as c3 FROM dual
UNION SELECT '2012-03-01 02:00:00', 3, 2 FROM dual
UNION SELECT '2012-03-01 01:00:00', 2, 1 FROM dual ) AS t2
WHERE t2.c2 = t1.c2
GROUP BY c1, c2;
EXECUTE s1;

For the last statement we have:
MariaDB [test]> EXECUTE s1;
--------------------------------

--------------------------------

--------------------------------
though the expected result is:
MariaDB [test]> EXECUTE s1;
--------------------------------

--------------------------------

--------------------------------

(see also bug #13805127 for mysql-5.5)