issue

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Ember Twiddle',
  
});

controllers.example1.js

import Ember from 'ember';

export default Ember.Controller.extend({
   hints: Ember.computed(function() {
    return ['hint #1', 'hint #2'];
  })
});

router.js

import EmberRouter from '@ember/routing/router';
import config from './config/environment';

const Router = EmberRouter.extend({
  location: 'none',
  rootURL: config.rootURL
});

Router.map(function() {
  this.route('example1');

});

export default Router;

routes.index.js

import Ember from 'ember';

export default Ember.Route.extend({
 beforeModel() {
   window.customElements.define('foo-bar', fooBar);
   this.transitionTo('example1');
 }
});

function fooBar() {
  return Reflect.construct(HTMLElement, [], fooBar);
}

fooBar.prototype = Object.create(HTMLElement.prototype, {
    constructor: { value: fooBar },
    hints: {value: undefined, writable:true }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>

templates.example1.hbs

<div>
Inspect the DOM to get foo.
<p>Why does hints bind as an attribute and not a property?</p>
<foo-bar onclick={{hints}} />
</div>

twiddle.json

{
  "version": "0.15.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.js",
    "ember": "3.4.3",
    "ember-template-compiler": "3.4.3",
    "ember-testing": "3.4.3"
  },
  "addons": {
    "ember-data": "3.4.2"
  }
}