Computed Model Weirdness

controllers.application.js

import OtherController from './other';

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

controllers.other.js

import Ember from 'ember';

export default Ember.Controller.extend({
  a: Ember.observer('model.[]', function() {
    console.log('observer', this.get('model.length'));
  }),
  
  b: Ember.computed('model.[]', function() {
    console.log('computed', this.get('model.length'));
    return this.get('model');
  })
});

routes.application.js

import OtherRoute from './other';

export default OtherRoute.extend();

routes.other.js

import Ember from 'ember';

export default Ember.Route.extend({
  model() {
    return Ember.RSVP.resolve([ 1, 2, 3 ]);
  },
  
  renderTemplate() {
    this.render('other');
  }
});

templates.application.hbs

<h1>Welcome to {{appName}}</h1>
<br>
<br>
{{b.length}}
<br>
<br>

templates.other.hbs

{{b.length}}

twiddle.json

{
  "version": "0.10.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.6.0",
    "ember-data": "2.6.1",
    "ember-template-compiler": "2.6.0"
  },
  "addons": {}
}