Why does GAPPS return 504 Gateway Error to a Zendesk webhook request?

Question

I have the following flow:

1. Zendesk triggers a webhook that makes a POST request with JSON object of 7 keys to my Google Apps Script.
2. Based on a value of a certain key, the script defines the Spreadsheet ID and simply appends a new row with other request body values.

It works almost always, except for random cases when Google turns back with 504 Gateway Error, after 12 seconds (it's a standard Zendesk window for 'retry'). Zendesk makes multiple retries for up to 5 times, and at last it always ends up with 200 OK. However, as I understand, the GAPPS stacks these calls and execute them all, which ends with up to 5 new similar rows being added to the doc.

My question is, what could cause such random delays in script execution, and how do I omit them?

I'm leaving my code for better understanding:

function doPost(request) {

  var jsonString = request.postData.getDataAsString();
  var jsonData = JSON.parse(jsonString);

  var ticketId =  jsonData.id;
  var comment = jsonData.comment;
  var game = jsonData.game;
  var locale = jsonData.locale;
  var assignee = jsonData.assignee;
  var priority = jsonData.priority;
  var ssId = getSpreadsheetId(locale);
  var sheetName = getSheetName(comment, priority);
  var updated = new Date();
  var sheet = SpreadsheetApp.openById(ssId).getSheetByName(sheetName);
  sheet.appendRow([ticketId, comment, "", game, assignee, priority, "New", updated]);
}

function getSheetName(comment, priority) {
  if ((comment.search("#to translate") >= 0) && priority == "Urgent") {
    var sheetName = "SEND URG";
  } else if (comment.search("#to translate") >= 0) {
    var sheetName = "SEND";
  } else if (comment.search("#to understand") >= 0) {
     var sheetName = "UNDERSTAND";
  }
  return sheetName;
}

function getSpreadsheetId(locale) {
  switch (locale) {
    case "Deutsch":
      var spreadSheetId = "german_doc_id";
      break;
    case "Italiano":
      var spreadSheetId = "italian_doc_id";
      break;
  }
  return spreadSheetId;
}

My assumption was that it was a matter of the size of the document that's being opened with SpreadsheetApp.openById(id).openByName(name). However, after clearing all the documents completely, the issue still persists. The documents sheets are approximately ~200 rows at the moment, not more than 5 sheets per doc.