Unable to retrieve Service through CustomFieldValue

[Jackson Tong]

I would want to access my product moduleConfigOption1 inside CustomFieldSave hook

add_hook('CustomFieldSave', 1, function($vars) {
  $customFieldValue = \WHMCS\CustomField\CustomFieldValue::firstWhere([
    'fieldid' => $vars['fieldid'],
    'relid' => $vars['relid']
  ]);

  // this works
  $client = $customFieldValue->client;
  $customField = $customFieldValue->customField;
  
  $customFieldValue->service; // this is null
  $customField->product; // null here too
});

According to the document https://classdocs.whmcs.com/8.1/WHMCS/CustomField/CustomFieldValue.html CustomFieldValue has a public property name service, but it's null in my code so I can't do this:

Quote

$customFieldValue->service->product->moduleConfigOption1