Form textfield AJAX validation "deletes" injected services to form

I’m trying to create a form that has two services injected (for access to an API) and a textfield with an AJAX callback. Here it is:

public function buildForm(array $form, FormStateInterface $form_state) {
    $form['vat_number'] = [
        '#title' => $this->t('VAT Number'),
        '#type' => 'textfield',
        '#default_value' => isset($address['vat_number']) ? $address['vat_number'] : '',
        '#states' => [
          'visible' => [
            ':input[name="is_business"]' => ['checked' => TRUE],
          ],
          'required' => [
            ':input[name="is_business"]' => ['checked' => TRUE],
          ],
        ],
        '#ajax' => [
          'callback' => '::validateVatAjax',
          'event' => 'change',
          'url' => Url::fromRoute('shop_product.digital_river_payment_form'),
          'options' => [
            'query' => [
              FormBuilderInterface::AJAX_FORM_REQUEST => TRUE,
            ],
          ],
        ],
        '#suffix' => '</div>',
      ];
    }
.....
}
public function validateVatAjax(array &$form, FormStateInterface $form_state) {$response = new AjaxResponse();
    $message = '<div id="errors" class="alert alert-danger" role="alert">';

    if (!$this->customerService->validateCustomerTaxFormat($form_state->getValue('vat_number'))) {
      $message .= $this->customerService->$schema['properties']['value']['description'];
      $message .= '</div>';
    }
    else {
      $message = '<div id="errors" class="alert alert-danger hide" role="alert"></div>';
    }
    $response->addCommand(new ReplaceCommand('#errors', $message));
    $form_state->setRebuild(FALSE);
    return $response;
  }

public function validateForm(array &$form, FormStateInterface $form_state) {
if ($form_state->getValue('is_business') == 1) {
        $vat_number = $form_state->getValue('vat_number');
        if (empty($vat_number)) {
          $form_state->setErrorByName('vat_number', $this->t('Please enter your tax number.'));
        }
        elseif (!$this->customerService->validateCustomerTaxFormat($form_state->getValue('vat_number'))) {
          $schema = $this->customerService->getSchema();
            $form_state->setErrorByName('vat_number', $schema['properties']['value']['description']);
          }
          else {
            $form_state->setErrorByName('vat_number', $this->t('Please enter your VAT number including the two letter country code.'));
          }
        }
      }
}

I omitted some of the code but my problem is that on the second AJAX call to the callback, it fails because the customerService is null. Aside from that, I don’t understand why validateForm() is called on the textfield input change. Is this normal for Drupal?

Thanks in advance.