Parse-Server, email-verification bug?

Question

I have been stuck longer than I would like, with a problem related to email verification when creating an account on Parse-Server (/Heroku). Though I made a few post on the issue, I was not lucky enough (or maybe did not formulate things clearly enough) to get significant help. So I decided to do it all over again, this time giving a precise step by step way to reproduce the bug, for anyone interested in taking a close look. If what I do is right from start to end, then there must be a bug. If on the other hand I do something wrong (this is most probably the case), I hope somebody can point out the mistake.

Here is the process, start by creating an app on heroku, using the following commands at the terminal:

git clone https://github.com/parse-community/parse-server-example.git
mv parse-server-example linkbugapp808
cd linkbugapp808/
npm install @parse/simple-mailgun-adapter  --save
heroku create linkbugapp808

heroku addons:create mongolab:sandbox
heroku config:set APP_ID=ABCDEF-12345678:xytzt_SSTTJJZ
heroku config:set MASTER_KEY=MMMMM-87878787:wwyyssaa_PPGHYU
heroku config:set SERVER_URL=https://linkbugapp808.herokuapp.com/
heroku config:set PARSE_PUBLIC_SERVER_URL=https://linkbugapp808.herokuapp.com

Of course if the name I use "linkbugapp808" is taken or you don't like it you may choose another one.

Set the index.js file as this (fixing the mailgun parameters that need to be fixed, to fit you environment):

var express = require('express');
var ParseServer = require('parse-server').ParseServer;
var path = require('path');
var mongo = require('mongodb');
var MongoClient = mongo.MongoClient;

var databaseUri = process.env.DATABASE_URI || process.env.MONGODB_URI;

if (!databaseUri) {
  console.log('DATABASE_URI not specified, falling back to localhost.');
}

var api = new ParseServer({
  databaseURI: databaseUri,
  cloud: process.env.CLOUD_CODE_MAIN || __dirname + '/cloud/main.js',
  appId: process.env.APP_ID,
  masterKey: process.env.MASTER_KEY,
  serverURL: process.env.SERVER_URL,
  publicServerURL: process.env.PARSE_PUBLIC_SERVER_URL,
  appName: 'LinkBugApp',
  verifyUserEmails: true,
  emailAdapter: {
    module: '@parse/simple-mailgun-adapter',
    options: {
      fromAddress: 'from@somemail.com',
      domain: 'some.domain',
      apiKey: 'key-apiKey-mailgun-apiKey'
    }
  }
});

var app = express();

// Serve static assets from the /public folder
app.use('/public', express.static(path.join(__dirname, '/public')));
app.set('port', (process.env.PORT || 5000));
app.use(express.static(__dirname + '/public'));
app.set('views', __dirname + '/views');
app.set('view engine', 'ejs');

// Serve the Parse API on the /parse URL prefix
var mountPath = process.env.PARSE_MOUNT || '/parse';
app.use(mountPath, api);

// Parse Server plays nicely with the rest of your web routes
app.get('/', function(req, res) {
  res.status(200).send('I dream of being a website.  Please star the parse-server repo on GitHub!');
});

// There will be a test page available on the /test path of your server url
// Remove this before launching your app
app.get('/test', function(req, res) {
  res.sendFile(path.join(__dirname, '/public/test.html'));
});

var port = process.env.PORT || 1337;
var httpServer = require('http').createServer(app);
httpServer.listen(port, function() {
    console.log('parse-server-example running on port ' + port + '.');
});

// This will enable the Live Query real-time server
ParseServer.createLiveQueryServer(httpServer);

Next run the following command in the terminal (inside the linkbugapp808 root folder):

git add . && git commit -m "update linkbugapp808" && git push heroku master

At this point the app is created on Heroku and ready to go.

Then from an iOS app create an account on the Parse-Server we just set above.

All seems to go fine.

The user for whom the account was created will receive a mail similar to this one:

Hi,

You are being asked to confirm the e-mail address usermail@xmail.com with LinkBugApp

Click here to confirm it:
https://linkbugapp808.herokuapp.com/apps/ABCDEF-12345678:xytzt_SSTTJJZ/verify_email?token=SiYyk9NgVkcwhXXWlEdEUTjyz&username=ausrnamex

When clicking the confirmation link inside the mail this is what one can see (not exactly what is expected from a sign up confirmation link):

Cannot GET /apps/ABCDEF-12345678:xytzt_SSTTJJZ/verify_email?token=SiYyk9NgVkcwhXXWlEdEUTjyz&username=ausrnamex 

I have tried with several browsers, but the result is identical.

Why do we get into this situation?