New Twiddle

application.controller.js

import Ember from 'ember';

export default Ember.Controller.extend({
  appName:'Set/Get CP init issue?',
});

application.template.hbs

<h1>{{appName}}</h1>
<br>
<br>
{{my-component}}

my-component.component.js

import Ember from 'ember';

const width = 10;

export default Ember.Component.extend({
  height: null,
  width: width,
  
  area: Ember.computed('height', {
    get(key) {
      return this.get('height') * width; 
    },

    set(key, value) {
      this.set('height', value/width);
    },
  }),
  
  setup: Ember.on('init', function() {
    this.set('area', 100);

    console.log('>>> height is changed to 10:', this.get('height'));
    console.log('>>> but area should be 100 but is undefined:', this.get('area'));
    
    this.set('height', 11);
    console.log('>>> area after mutating height', this.get('area'));    
  }),
});

my-component.template.hbs

height: {{height}}
<br>
width: {{width}}
<br>
area: {{area}}
<br>
<br>
Check the console and note that 'area' is undefined after 'setting' it.

twiddle.json

{
  "version": "0.4.10",
  "dependencies": {
    "jquery": "https://cdnjs.cloudflare.com/ajax/libs/jquery/1.11.3/jquery.js",
    "ember": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.9/ember.debug.js",
    "ember-data": "https://cdnjs.cloudflare.com/ajax/libs/ember-data.js/1.13.11/ember-data.js",
    "ember-template-compiler": "https://cdnjs.cloudflare.com/ajax/libs/ember.js/1.13.9/ember-template-compiler.js"
  }
}