New Twiddle

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
	noop() {},
  
  checked: true,
  
  setToTrue() {
    this.set('checked', true);
  },
  
  setToFalse() {
    this.set('checked', false);
  },
});

templates.application.hbs

<h1>Checked should be: {{this.checked}}</h1>

<div>{{input type="checkbox" checked=this.checked change=this.noop}}</div>

<div><button {{action this.setToTrue}}>Set to true</button></div>

<div><button {{action this.setToFalse}}>Set to false</button></div>

<ol>
  <li>Click checkbox to unselect</li>
  <li>Click set to true</li>
  <li>Expected: change checkbox to selected, Actual: nothing happens</li>
</ol>

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"
  }
}