Window function produces incorrect value

create table t0 (a int);

insert into t0 values (0),(1),(2),(3),(4),(5),(6),(7),(8),(9);

create table t2 (part_id int, pk int, a int);

insert into t2 select if(a<5, 0, 1), a, if(a<5, NULL, 1) from t0;

select * from t2;

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

| part_id | pk   | a    |

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

|       0 |    0 | NULL |

|       0 |    1 | NULL |

|       0 |    2 | NULL |

|       0 |    3 | NULL |

|       0 |    4 | NULL |

|       1 |    5 |    1 |

|       1 |    6 |    1 |

|       1 |    7 |    1 |

|       1 |    8 |    1 |

|       1 |    9 |    1 |

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

select 

  part_id, pk, a,

  count(a) over (partition by part_id order by pk

                 rows between 1 preceding and 1 following) as CNT

from t2;

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

| part_id | pk   | a    | CNT |

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

|       0 |    0 | NULL |   0 |

|       0 |    1 | NULL |   0 |

|       0 |    2 | NULL |   0 |

|       0 |    3 | NULL |   0 |

|       0 |    4 | NULL |   0 |

|       1 |    5 |    1 |   0 |

|       1 |    6 |    1 |   0 |

|       1 |    7 |    1 |   0 |

|       1 |    8 |    1 |   0 |

|       1 |    9 |    1 |   0 |

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