START/STOP ALL SLAVES does not return access errors

MariaDB [(none)]> show grants;

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

| Grants for u@localhost                |

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

| GRANT USAGE ON *.* TO `u`@`localhost` |

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

1 row in set (0.000 sec)

MariaDB [(none)]> stop slave;

ERROR 1045 (28000): Access denied for user 'u'@'localhost' (using password: NO)

MariaDB [(none)]> stop all slaves;

Query OK, 0 rows affected (0.000 sec)