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;
+---------------------+----+-----------+
| c1 | c2 | count(c3) |
+---------------------+----+-----------+
| 2012-03-01 01:00:00 | 3 | 2 |
| 2012-03-01 02:00:00 | 3 | 1 |
+---------------------+----+-----------+
though the expected result is:
MariaDB [test]> EXECUTE s1;
+---------------------+----+-----------+
| c1 | c2 | count(c3) |
+---------------------+----+-----------+
| 2012-03-01 01:00:00 | 2 | 1 |
| 2012-03-01 01:00:00 | 3 | 1 |
| 2012-03-01 02:00:00 | 3 | 1 |
+---------------------+----+-----------+

(see also bug #13805127 for mysql-5.5)