Biblioteca Mechado Pago não permite enviar novo campo do formulário

const mp = new MercadoPago('SAND_BOX');

var cardNumber = document.querySelector('#form-checkout__cardNumber');
var brand = document.querySelector('.brand');

cardNumber.addEventListener("keyup", function() {
  if (cardNumber.value.length < 6) {
      brand.style.display = "none";
  }
});

const cardForm = mp.cardForm({
  amount: "100.5",  //Desnecessário aqui, remover
  autoMount: true,  //Desnecessário aqui, remover
  form: {
    id: "form-checkout",
    cardholderName: {
      id: "form-checkout__cardholderName",
      placeholder: "Titular do cartão",
    },
    cardholderEmail: {
      id: "form-checkout__cardholderEmail",
      placeholder: "E-mail",
    },
    cardNumber: {
      id: "form-checkout__cardNumber",
      placeholder: "Número do cartão",
    },
    cardExpirationMonth: {
      id: "form-checkout__cardExpirationMonth",
      placeholder: "Mês de vencimento",
    },
    cardExpirationYear: {
      id: "form-checkout__cardExpirationYear",
      placeholder: "Ano de vencimento",
    },
    securityCode: {
      id: "form-checkout__securityCode",
      placeholder: "Código de segurança",
    },
    installments: {
      id: "form-checkout__installments",
      placeholder: "Parcelas",
    },
    identificationType: {
      id: "form-checkout__identificationType",
      placeholder: "Tipo de documento",
    },
    identificationNumber: {
      id: "form-checkout__identificationNumber",
      placeholder: "Número do documento",
    },
    issuer: {
      id: "form-checkout__issuer",
      placeholder: "Banco emissor",
    },
    order: {
      id: "form-checkout__order",
      placeholder: "Id do pedido",
    },
  },
  callbacks: {
    onFormMounted: error => {
      if (error) return console.warn("Form Mounted handling error: ", error);
      console.log("Form mounted");
    },
    onSubmit: event => {
      event.preventDefault();
      console.log(cardForm.getCardFormData());
      const {
        paymentMethodId: payment_method_id,
        issuerId: issuer_id,
        cardholderEmail: email,
        amount,
        token,
        installments,
        identificationNumber,
        identificationType,
        order : order, //necessário enviar mas não está chegando no PHP

      } = cardForm.getCardFormData();

      fetch("/loja/process_payment", {
        method: "POST",
        headers: {
          "Content-Type": "application/json",
        },
        body: JSON.stringify({
          token,
          issuer_id,
          payment_method_id,
          transaction_amount: Number(amount),
          installments: Number(installments),
          description: "Descrição do produto",
          email,
          type: identificationType,
          number: identificationNumber,
          order: order,  //necessário enviar mas não está chegando no PHP
          //order: $("#form-checkout__order").value,
        }),
      }).then(function(response) {
        response.json().then(function(json) {
          
          if (json.status == "approved") {
            window.location.href = "ok.php";
            console.log(json.status);
          } else 
            console.log(json);
        });
      });
    },
    onPaymentMethodsReceived: (error, paymentMethods) => {
        if (error) return console.warn('paymentMethods handling error: ', error);
        if (cardNumber.value.length >= 6) {
            brand.style.display = "flex";
            brand.innerHTML="<img src='" + paymentMethods[0].thumbnail + "' alt='Bandeira do Cartão'>";
        } 
    },
    onFetching: (resource) => {
      console.log("Fetching resource: ", resource);
      // Animate progress bar
      const progressBar = document.querySelector(".progress-bar");
      progressBar.removeAttribute("value");

      return () => {
        progressBar.setAttribute("value", "0");
      };
    },
  },
});