NOT IN subquery for some SELECTs and DMLs does not work as expected

MariaDB [test]> create table t1 (a int, b varchar(100))engine=columnstore;

Query OK, 0 rows affected (0.213 sec)

MariaDB [test]> create table i1 (a int, b varchar(100));

Query OK, 0 rows affected (0.007 sec)

MariaDB [test]> insert into t1 values (1, 'val'), (2, 'val'), (null, 'nullval'), (4, 'val');

Query OK, 4 rows affected (0.507 sec)

Records: 4  Duplicates: 0  Warnings: 0

MariaDB [test]> insert into i1 values (1, 'val'), (3, 'val'), (null, 'nullval'), (4, 'val');

Query OK, 4 rows affected (0.004 sec)

Records: 4  Duplicates: 0  Warnings: 0

-- Works as expected

MariaDB [test]> select * from i1 where a not in (select a from t1);

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

| a    | b    |

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

|    3 | val  |

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

1 row in set (0.128 sec)

-- Doesn't work as expected

MariaDB [test]> select * from t1 where a not in (select a from i1);

Empty set (0.038 sec)

-- Doesn't work as expected

MariaDB [test]> update i1 set b='Update' where a not in (select a from t1);

Query OK, 0 rows affected (0.063 sec)

Rows matched: 0  Changed: 0  Warnings: 0

MariaDB [test]> select * from i1;

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

| a    | b       |

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

|    1 | val     |

|    3 | val     |

| NULL | nullval |

|    4 | val     |

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

4 rows in set (0.001 sec)

-- Doesn't work as expected, updates more records than required

MariaDB [test]> update t1 set b='Update' where a not in (select a from i1);

Query OK, 3 rows affected (0.447 sec)

Rows matched: 3  Changed: 3  Warnings: 0

MariaDB [test]> select * from t1;

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

| a    | b       |

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

|    1 | Update  |

|    2 | Update  |

| NULL | nullval |

|    4 | Update  |

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

4 rows in set (0.028 sec)

-- works as expected

MariaDB [test]> update t1 set b='Update2' where a not in (select a from t1);

Query OK, 0 rows affected (0.247 sec)

Rows matched: 0  Changed: 0  Warnings: 0

MariaDB [test]> select * from t1;

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

| a    | b       |

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

|    1 | Update  |

|    2 | Update  |

| NULL | nullval |

|    4 | Update  |

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

4 rows in set (0.016 sec)