ROW_NUMBER is not set and differs from the message upon ER_WRONG_VALUE_COUNT_ON_ROW for the 1st row

create or replace table t (a int);

insert into t values (1,2),(3);

get diagnostics condition 1 @n= row_number, @m= message_text;

select @n, @m;

MariaDB [test]> select @n, @m;

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

| @n   | @m                                              |

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

|    0 | Column count doesn't match value count at row 1 |

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

1 row in set (0.001 sec)

MariaDB [test]> insert into t values (1),(2,3);

ERROR 1136 (21S01): Column count doesn't match value count at row 2

MariaDB [test]> get diagnostics condition 1 @n= row_number, @m= message_text;

Query OK, 0 rows affected (0.000 sec)

MariaDB [test]> select @n, @m;

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

| @n   | @m                                              |

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

|    2 | Column count doesn't match value count at row 2 |

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

1 row in set (0.001 sec)