post.comments.@each.id observers are not called when a new post comment is saved

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Ember Twiddle',
  
  init() {
    this._super(...arguments);
    
    $.mockjax({
      url: '/posts/1',
      responseText: { 
        'data': { 
          'id': '1', 
          'type': 'post', 
          'attributes': { 
            'name': "1st post" 
          },
          'relationships': {
            'comments': {
              'data': [],
              'links': {
                'related': "/posts/1/comments",
                'self': "/posts/1/relationships/comments"
              }
            }
          }
        }
      }
    });
    $.mockjax({
      url: '/comments',
      responseText: { 
        'data': { 
          'id': '1', 
          'type': 'comment'
        },
        'relationships': {
          'post': {
            'links': {
              'related': "/comments/1/game",
              'self': "/comments/1/relationships/game"
            }
          }
        }
      }
    });
  },
  
  post: Ember.computed(function() {
    return this.store.findRecord('post', '1');
  }),

  postCommentsIdChanged: Ember.observer('post.comments.@each.id', function() {
    console.log('post.comments.@each.id changed');
  }),

  actions: {
    buildComment() {
      this.get('post').then((post) => {
        let comment = this.store.createRecord('comment', { post: post });
        this.set('newComment', comment);
      });
      
    },
    saveComment() {
      let comment = this.get('newComment');
      console.log('will save, post.comments.@each.id should change soon');
      comment.save().then( () => {
        let post = this.get('post');
        console.log('did save, post.comments.@each.id observers should be called');
        console.log('posts.comments.length: ' + post.get('comments.length'));
        console.log('posts.comments ids: [' + post.get('comments').mapBy('id') + ']');
      });
    }
  }
});

application.template.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{outlet}}
<br>
<br>
<h3>{{post.name}}</h3>
<ul>
  {{#each post.comments as |comment|}}
    <li></li>
  {{/each}}
</ul>
<button {{action "buildComment"}}>Build comment</button>
<button {{action "saveComment"}}>Save comment</button>

models.comment.js

import DS from 'ember-data';

export default DS.Model.extend({
  content: DS.attr(),
  post: DS.belongsTo('post')
});

models.post.js

import DS from 'ember-data';

export default DS.Model.extend({
  name: DS.attr(),
  comments: DS.hasMany('comment')
});

twiddle.json

{
  "version": "0.4.17",
  "EmberENV": {
    "FEATURES": {}
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.2.0/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/2.2.0/ember-template-compiler.js",
    "jquery-mockjax": "https://cdnjs.cloudflare.com/ajax/libs/jquery-mockjax/1.6.2/jquery.mockjax.js"
  }
}