Ember CLI: Broccoli Funnel not copying files

I have several JSON files in my Ember project in a directory called data within the app directory:

I am attempting to copy these files into another data directory within the public directory using a Broccoli funnel. However, when running ember serve, the files are not copied and no errors are reported. After reading the answer to this question, I started using broccoli-merge-trees which did not fix the issue.

/* eslint-env node */
'use strict';

const EmberApp = require('ember-cli/lib/broccoli/ember-app');
const Funnel = require('broccoli-funnel');
const MergeTrees = require('broccoli-merge-trees');

module.exports = function(defaults) {
  let app = new EmberApp(defaults, {
    // Add options here
  });

  var json = new Funnel('app/data', {
      destDir: 'public/data',
      include: ['*.json']
  });

  return new MergeTrees([json, app.toTree()]);
};