Observers for sub-properties don't work when using notifyPropertyChange

console.js

X.b.get('prop') // => 0
X.b.setA(X.o)
X.b.get('prop') // => 0
X.b.get('a').get('prop') // => 1
X.b.notifyPropertyChange('prop')
X.b.get('prop') // => 1

x.js

X = {
  a: SC.Object.create({
    prop: 1
  }),
  
  b: SC.Object.create({
    a: function() {
      return this._a;
    }.property().cacheable(),
    
    prop: function() {
      a = this.get('a');
      if (SC.none(a)) return 0;
      else return a.get('prop');
    }.property('*a.prop').cacheable(),
    
    setA: function(a) {
      this._a = a; this.notifyPropertyChange('a');
    }
  })
}