Apple Pay JS completeMerchantValidation results in oncancel event

Hello, I am trying to implement Apple Pay for web on our site and seeing some odd behavior.

I see onvalidatemerchant and completeMerchantValidation being called, but a fairly opaque oncancel event immediately follows.

• I am able to load the Apple Pay JS script and instantiate an ApplePaySession on the client.
• my server seems to be communicating effectively with the client, and returns a payment session from https://apple-pay-gateway-cert.apple.com/paymentservices/paymentSession.
• I have checked the hash of the merchantIdentifier and it matches what is returned in the payment session
• I have implemented all of the ApplePaySession event handler methods (onvalidatemerchant, onpaymentauthorized, onpaymentmethodselected, etc.) -- only onvalidatemerchant and oncancel are called
• I have registered both <subdomain>.<mydomain>.com and <mydomain>.com, where the code is deployed
• I have created a sandbox tester Apple account and added a test card to it, and am running the test from an iPad that is logged into the sandbox tester account
• the data being passed to completeMerchantValidation is a JS object, not JSON, which threw a TypeError

Here is the deployed code.

applePaySession = new ApplePaySession(3, request);

applePaySession.onvalidatemerchant = async function (event) {
  try {
    const { data } = await getApplePayPaymentSession();
    applePaySession.completeMerchantValidation(data);
  } catch (e) {
    console.log(e);
  }
};

applePaySession.onpaymentauthorized = function(event) {
  const result = {
    "status": ApplePaySession.STATUS_SUCCESS
  };
  applePaySession.completePayment(result);
  applePaySession = null;
};

applePaySession.onpaymentmethodselected = event => {
  // No updates or errors are needed, pass an empty object.
  const update = {};
  applePaySession.completePaymentMethodSelection(update);
};

applePaySession.onshippingmethodselected = event => {
  // No updates or errors are needed, pass an empty object.
  const update = {};
  applePaySession.completeShippingMethodSelection(update);
};
applePaySession.onshippingcontactselected = event => {
  const update = {};
  applePaySession.completeShippingContactSelection(update);
};

applePaySession.oncancel = function(event) {
  console.log('oncancel', event);
};

applePaySession.begin();

Any guidance would be highly appreciated!