First Name and Last Name are not getting copied to user data

Hi,

I have created a Registration form, and custom fields for First Name and Last Name.
I folllowed the guide (https://metabox.io/create-wordpress-custom-user-profile-page-in-frontend/#more-17815) as much as possible.
Yet, the page looks different, and the first and last name are not copied to the user data.

This is the registration page: https://share.getcloudapp.com/NQuJmqPr

This is the shortcode: [mb_user_profile_register id="user-profile-default-fields" label_submit="Register" confirmation="Your account has been created successfully!" role="customer"]

This is the code:

$meta_boxes[] = [
    'title'      => esc_html__( 'Personal Info', 'text-domain' ),
    'id'         => 'user-profile-default-fields',
    'post_types' => ['post'],
    'context'    => 'normal',
    'priority'   => 'high',
    'fields'     => [
        [
            'id'       => $prefix . 'first_name',
            'type'     => 'text',
            'name'     => esc_html__( 'First Name', 'text-domain' ),
            'required' => 1,
        ],
        [
            'id'   => $prefix . 'last_name',
            'type' => 'text',
            'name' => esc_html__( 'Last Name', 'text-domain' ),
        ],
    ],
];

return $meta_boxes;

Please note that it is a subsite of multisite installation.