cp not updating in certain use cases

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  comments: Ember.computed('model.comments.[]', function() {
    return window.comments;
  })
});

models.comment.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  post: belongsTo('post'),
  content: null,
  characters: Ember.computed('content', function() {
    let content = this.get('content');
    
    if(!content) {
      content = '';
    } 
    
    return content.length;
  })
});

models.post.js

import Model from "ember-data/model";
import attr from "ember-data/attr";
import { belongsTo, hasMany } from "ember-data/relationships";

export default Model.extend({
  comments: Ember.computed(function() {
  	return window.comments; 
  }),
  characters: Ember.computed('[email protected]', function() {
    let characters = 0;
  	window.comments.forEach(function(comment) {
      characters += comment.get('characters');
    });
    
    return characters + 10;
  })
});

routes.application.js

import Ember from 'ember';

export default Ember.Route.extend({
  model: function() {
 	 return this.store.createRecord('post');
  },
  afterModel: function() {
  	window.comments = [this.store.createRecord('comment')];
  }
});

templates.application.hbs

Total characters: {{model.characters}}

<br /><br />

{{#each comments as |comment|}}
	<div>
    content: {{input value=comment.content}}
  </div>
  
  <div>
    characters: {{comment.characters}}
	</div>
  
  <br /><br />
{{/each}}

twiddle.json

{
  "version": "0.8.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.5.1",
    "ember-data": "2.5.1",
    "ember-template-compiler": "2.5.1"
  }
}