Custom rwmb_profile_register_fields do not save to usermeta?

Hi there,
I am using the MB AIO plugin (3.3.4) with the MB User Profile extension. Can you confirm the expected "saving" behaviour when adding additional "Custom" registration fields using the rwmb_profile_register_fields filter. For example using:

add_filter( 'rwmb_profile_register_fields', 'custom_user_registration_fields' );
function custom_user_registration_fields( $fields ) {
    $fields = [
        [
         // Additional Custom Registration Field
        'name'     => __( 'First name', 'understrap' ),
        'id'       => 'first_name',
        'type'     => 'text',
        'required' => true,
        ],
        // Other fields etc....
    ];
    return $fields;
}

This custom registration form implementation works but the custom "first_name" field does NOT get saved to the usermeta? Is this expected behaviour?

Please confirm?

Cheers 🙂