Primitive value set only change once

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName: 'Welcome Ember',
  
  test1: 'primitive',
  
  test2: {
    value: null
  },
  
  detectValue1: Ember.observer('test1', function() {
		alert('changed')
	}),
  
  detectValue2: Ember.observer('test2', function() {
		alert('changed')
	}),

  detectValue3: Ember.observer('test2.value', function() {
		alert('changed')
	}),
  
  actions: {
    setValue1() {
      this.set('test1', 0);
    },
    
    setValue2() {
      this.set('test2', { value: 0 });
    },
    
    setValue3() {
      this.set('test2.value', 0);
    }
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
<button type="button" onclick={{action "setValue1"}}>TEST 1</button>

<button type="button" onclick={{action "setValue2"}}>TEST 2</button>

<button type="button" onclick={{action "setValue3"}}>TEST 3</button>

twiddle.json

{
  "version": "0.12.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.12.0",
    "ember-template-compiler": "2.12.0",
    "ember-testing": "2.12.0"
  },
  "addons": {
    "ember-data": "2.12.1"
  }
}