Bug: The custom error config of validation is not working

[isiungk]

Describe the bug
According to the document description(/docs/libraries/validation.html#id13),
I set custom error message as the labeled style in "\Config\Validation",

// App\Config\Validation.php

public $login = [
    'username' => [
        'label'  => 'Username',
        'rules'  => 'required',
        'errors' => [
            'required' => 'custom username required error msg.',
        ],
    ],
    'password' => [
        'label'  => 'Password',
        'rules'  => 'required',
        'errors' => [
            'required' => 'custom password required error msg.',
        ],
    ]
];

Run the script below,

// App\Controllers\SomController.php

$validation =  \Config\Services::validation();
$validation->reset()->setRuleGroup('login')->run();
$errors = $validation->getErrors();
print_r($errors);

And I lost the custom error and got the default error message instead.

Array
(
    [username] => The Username field is required
    [password] => The Password field is required.
)

CodeIgniter 4 version
CodeIgniter 4.0.2

Affected module(s)
\App\Controller

Context

• OS: [CentOS7]
• Web server [Apache/2.4.41 (codeit) OpenSSL/1.1.1d PHP/7.4.3]
• PHP version [7.4.3]