Wrong result from query with NOT IN predicate in WHERE

create table t1 (a int, b int);

insert into t1 values 

 (1,2), (4,6), (9,7),(1,1), (2,5), (7,8);

select * from t1

where (a,b) not in ((1,2),(8,9), (5,1));

set @@in_subquery_conversion_threshold= 2;

select * from t1

where (a,b) not in ((1,2),(8,9), (5,1));

MariaDB [test]> 

MariaDB [test]> select * from t1

    -> where (a,b) not in ((1,2),(8,9), (5,1));

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

| a    | b    |

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

|    4 |    6 |

|    9 |    7 |

|    1 |    1 |

|    2 |    5 |

|    7 |    8 |

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

MariaDB [test]> set @@in_subquery_conversion_threshold= 2;

MariaDB [test]> select * from t1

    -> where (a,b) not in ((1,2),(8,9), (5,1));

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

| a    | b    |

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

|    1 |    2 |

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