Query returns wrong result while using big_tables=1;

create table t1 (a int, b int) engine=MyISAM;

create table t2 (c int, d int) engine=MyISAM;

create table t3 (e int, f int) engine=MyISAM;

create table t4 (g int, h int) engine=MyISAM;

insert into t1 values (1,1),(2,2),(3,3),(2,2),(3,3);

insert into t2 values (2,2),(3,3),(5,5),(2,2),(2,2),(3,3);

insert into t3 values (4,4),(2,2),(2,2),(1,1),(3,3);

insert into t4 values (3,3),(3,3);

let $q=

    select * from t1

    INTERSECT ALL

    select * from t2

    UNION ALL

    select * from t3

    EXCEPT ALL

    select * from t1

    UNION

    select * from t4

    EXCEPT

    select * from t3

    UNION ALL

    select * from t1;

eval $q;

set big_tables=1;

eval $q;

MariaDB [test1]> select * from t1

    ->     INTERSECT ALL

    ->     select * from t2

    ->     UNION ALL

    ->     select * from t3

    ->     EXCEPT ALL

    ->     select * from t1

    ->     UNION

    ->     select * from t4

    ->     EXCEPT

    ->     select * from t3

    ->     UNION ALL

    ->     select * from t1;

+------+------+

| a    | b    |

+------+------+

|    2 |    2 |

|    2 |    2 |

|    1 |    1 |

|    3 |    3 |

|    3 |    3 |

+------+------+

5 rows in set (0.009 sec)

MariaDB [test1]> set big_tables=1;

Query OK, 0 rows affected, 1 warning (0.001 sec)

MariaDB [test1]>     select * from t1

    ->     INTERSECT ALL

    ->     select * from t2

    ->     UNION ALL

    ->     select * from t3

    ->     EXCEPT ALL

    ->     select * from t1

    ->     UNION

    ->     select * from t4

    ->     EXCEPT

    ->     select * from t3

    ->     UNION ALL

    ->     select * from t1;

Empty set (0.013 sec)

MariaDB [test1]>  select * from t1     INTERSECT ALL     select * from t2     UNION ALL     select * from t3     EXCEPT ALL     select * from t1     UNION     select * from t4     EXCEPT     select * from t3     UNION    (select * from t1);

+------+------+

| a    | b    |

+------+------+

|    2 |    2 |

|    1 |    1 |

|    3 |    3 |

+------+------+

3 rows in set (0.013 sec)

MariaDB [test1]>  select * from t1     INTERSECT ALL     select * from t2     UNION ALL     select * from t3     EXCEPT ALL     select * from t1     UNION     select * from t4     EXCEPT     select * from t3     UNION  all  (select * from t1);

Empty set (0.007 sec)