strange-issue

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
    init() {
      this._super(...arguments);
      this.set('items', []);
      Ember.run.later(() => this.set('items', [1]), 1000);
    },

    toString() {
      return this.get('length');
    },

    length: Ember.computed('items.length', function() {
      return this.get('items.length');
    }),
});

templates.application.hbs

length: "{{length}}"

twiddle.json

{
  "version": "0.10.6",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.9.0",
    "ember-data": "2.9.0",
    "ember-template-compiler": "2.9.0",
    "ember-testing": "2.9.0"
  },
  "addons": {}
}