\W client command does not work well with DELIMITER

I found this bug adding \W at the beginning of a batch file, but I can write a command-line test case:

MariaDB [stk_apalog]> \W

Show warnings enabled.

MariaDB [stk_apalog]> DELIMITER ||

MariaDB [stk_apalog]> CREATE DATABASE IF NOT EXISTS test;

    -> DO NULL;

    -> ||

Query OK, 1 row affected, 1 warning (0.00 sec)

Query OK, 0 rows affected (0.00 sec)

The || delimiter runs 2 statements in this case. If the first statement generates a warning, the warning does not appear. By deleting USE NULL and retrying, the warning appears.