Inserting NULL into base column breaks NATURAL_SORT_KEY column

create or replace table t (a varchar(8), b varchar(8) as (natural_sort_key(a)));

insert into t (a) values ('a2'),(NULL),('a11');

select * from t order by b;

select a, b from t order by b;

MariaDB [test]> select * from t order by b;

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

| a    | b    |

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

| NULL | NULL |

| a11  | NULL |

| a2   | a02  |

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

3 rows in set (0.001 sec)

MariaDB [test]> select a, b from t order by b;

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

| a    | b    |

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

| a2   | NULL |

| NULL | NULL |

| a11  | NULL |

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

3 rows in set (0.001 sec)