computed.sort issue

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  
  version: Ember.VERSION,
  
  sortBy11: [],
  sortBy12: ['a'],
  
  c1: [{a: 1}, {a: 2}, {a: 3}, {a: 4}, {a: 5}, {a: 6}, {a: 7}, {a: 8}, {a: 9}, {a: 10}],
  
  small1: Ember.computed.sort('c1', 'sortBy11'),
  small2: Ember.computed.sort('c1', 'sortBy12'),
  
  
  sortBy21: [],
  sortBy22: ['a'],
  
  c2: [{a: 1}, {a: 2}, {a: 3}, {a: 4}, {a: 5}, {a: 6}, {a: 7}, {a: 8}, {a: 9}, {a: 10}, {a: 11}, {a: 12}, {a: 13}, {a: 14}, {a: 15}, {a: 16}, {a: 17}, {a: 18}, {a: 19}, {a: 20}],
  
  big1: Ember.computed.sort('c2', 'sortBy21'),
  
  big2: Ember.computed.sort('c2', 'sortBy22')
});

application.template.hbs

<p><strong>Ember {{version}}</strong></p>

<table>
  <tr>
  	<td>
      Original "small" data:
      <ul>
        {{#each c1 as |item|}}
          <li>{{item.a}}</li>
        {{/each}}
      </ul>
    </td>
    <td>
      Sorted with <strong>empty</strong> `sort properties`:
      <ul>
        {{#each small1 as |item|}}
          <li>{{item.a}}</li>
        {{/each}}
      </ul>
    </td>
    <td>
      Sorted with <strong>filled</strong> `sort properties`:
      <ul>
        {{#each small2 as |item|}}
          <li>{{item.a}}</li>
        {{/each}}
      </ul>
    </td>
  </tr>
  <tr>
  	<td>
    Original "big" data:
      <ul>
        {{#each c2 as |item|}}
          <li>{{item.a}}</li>
        {{/each}}
    	</ul>
    </td>
    <td>
      Sorted with <strong>empty</strong> `sort properties`:
      <ul>
        {{#each big1 as |item|}}
          <li>{{item.a}}</li>
        {{/each}}
      </ul>
    </td>
    <td>
      Sorted with <strong>filled</strong> `sort properties`:
      <ul>
        {{#each big2 as |item|}}
          <li>{{item.a}}</li>
        {{/each}}
      </ul>
    </td>
  </tr>
</table>

{{outlet}}

twiddle.json

{
  "version": "0.6.1",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "release",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/2.3.3/ember-data.js",
    "ember-template-compiler": "release"
  }
}