SELECT...union/intersect/except...ORDER BY results are not consistent

SELECT...UNION...ORDER BY results are not consistent on both the engines Columnstore and Innodb. Due to this some tests are failing.

MariaDB [test]> CREATE TABLE t1 (t1_int INT, t1_char CHAR(5))ENGINE=Columnstore;
Query OK, 0 rows affected (0.127 sec)

MariaDB [test]> CREATE TABLE t2 (t2_int INT, t2_char CHAR(5))ENGINE=Columnstore;
Query OK, 0 rows affected (0.138 sec)

MariaDB [test]> INSERT INTO t1 VALUES (NULL, ''),(1, 'aaa'),(2, 'aaa'),(3, 'ccc'),(4, 'ddd'),(5, 'aaa'),(6, ''),(7, 'eee');
Query OK, 8 rows affected (0.824 sec)
Records: 8 Duplicates: 0 Warnings: 0

MariaDB [test]> INSERT INTO t2 VALUES (NULL, ''),(1, 'eee'),(3, 'ccc'),(5, 'jjj'),(6, ''),(7, 'lll'),(9, 'eee'),(11, 'nnn');
Query OK, 8 rows affected (0.503 sec)
Records: 8 Duplicates: 0 Warnings: 0

Run 1 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.153 sec)

Run 2 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.026 sec)

Run 3 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.025 sec)

Run 4 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.017 sec)

Run 5 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.349 sec)

Run 6 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.022 sec)

Run 7 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.018 sec)

Run 8 (here it started giving different result) -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.019 sec)

Run 9 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.022 sec)

Run 10 -
MariaDB [test]> SELECT t1_char, 'Character' AS Table1 FROM t1 UNION SELECT t2_int, 'Integer' AS Table2 FROM t2 ORDER BY 2 ASC;
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13 rows in set (0.024 sec)