Why does my custom Excel function never return a value ONLY on Windows?

I am building an Office Excel Add-in using the web add-in framework provided by Microsoft.
This add-in also includes a custom function. Currently, the custom function is working on Excel for Mac, Excel online (device agnostic), but not on Windows?

The add-in loads fine, and there are no obvious errors. But when the function is run (on Windows) it just says:
#BUSY and then resolves to #VALUE! and stays like that.

The code also works when using the Shared Runtime configuration, but that requires that we make all our Javascript compatible with IE, which is definitely a possibility – but I would like to know why the regular configuration is not working.

WISE is the Excel function.

function WISE(symbol, parameter, year, quarter) {
  var param = parameter.replace(/s/g, '').toLowerCase();
  param = param.replace('&', 'and');
  symbol = symbol.toUpperCase();
  if (quarter == null) {
    return getAnnualData(symbol, param, year);
  } 
}

function getVal(data, param) {
  var apiResponseDataFormatted = {};
  for (var key in data) {
    apiResponseDataFormatted[key.replace(/ /g, '').toLowerCase()] = data[key];
  }
  var newValue = apiResponseDataFormatted[param];
  if (newValue !== 0 && !newValue) {
    newValue = 'Unavailable';
  }
  return newValue;
}

function getAnnualData(symbol, parameter, year) {
  var apiPath = requestMap[parameter];
  var response = "";
  var url = URL_API + "/" + apiPath + "/" + symbol + "?apikey=" + api_key;

  var request = new XMLHttpRequest();
  request.open('GET', url, false);  // `false` makes the request synchronous
  request.send(null);

  if (request.status === 200) {// That's HTTP for 'ok'
    response = JSON.parse(request.responseText);
  }else{
    return "Request Error: " + request.status + " " + url;
  }

  var apiResponseData;
  var currentYear = new Date().getFullYear();
  if (year != null && year !== currentYear) {
    apiResponseData = response[currentYear - year - 1];
  } else {
    apiResponseData = response[0];
  }
  result = getVal(apiResponseData, parameter)

  return result;
}