Invisible Column Non-constant Default value results wrong values

MariaDB [d]> create table t2(a int , b int invisible default (a));

Query OK, 0 rows affected (0.01 sec)

MariaDB [d]> insert into t2 values(2);                                                                                                                

Query OK, 1 row affected (0.00 sec)

MariaDB [d]> select a,b from t2;                                                                                                                      

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

| a    | b    |

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

|    2 | NULL |

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

1 row in set (0.00 sec)