Unexpected observer behaviour

controllers.application.js

import Ember from 'ember';

export default Ember.Controller.extend({
  now: null,
  observerNow: null,
  _observerNow: Ember.observer('now', function() {
    this.set('observerNow', this.get('now'));
  }),
  tick() {
    Ember.run.later(this, function() {
      this.set('now', Date.now());
      this.tick();
    }, 1000);
  },
  init() {
    this.tick();
  }
});

templates.application.hbs

observerNow: {{observerNow}}

twiddle.json

{
  "version": "0.11.0",
  "EmberENV": {
    "FEATURES": {}
  },
  "options": {
    "use_pods": false,
    "enable-testing": false
  },
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "2.10.2",
    "ember-data": "2.11.0",
    "ember-template-compiler": "2.10.2",
    "ember-testing": "2.10.2"
  },
  "addons": {}
}