IssueTracker

As per the documentation:

Reply to a thread or do nothing. Add the messageReplyOption=REPLY_MESSAGE_OR_FAIL parameter to the webhook URL. Passing this URL parameter causes Chat to look for an existing thread using the specified threadKey. If one is found, then the message posts as a reply to that thread. If none is found, then the message isn't sent.

The messageReplyOption=REPLY_MESSAGE_OR_FAIL flag should never start a new thread - it should only reply to existing threads with the provided threadKey. However, this is not working. Even when the threadKey provided does not exist in the space, the webhook creates a new thread anyway.

For example:

var webhook = PropertiesService.getScriptProperties().getProperty('WEBHOOK');
var modifier = '&messageReplyOption=REPLY_MESSAGE_OR_FAIL';

var options = {
  "method": "post",
  "headers": { "Content-Type": "application/json; charset=UTF-8" },
  "payload": JSON.stringify({
    "text": 'more items ordered yaay',
    "thread": { 
      "threadKey": Utilities.getUuid(); // completely unique threadKey 
     }
   })
};

UrlFetchApp.fetch(webhook + modifier, options);

The above ends up creating a new thread, even when the threadKey does not exist in the space.