Ember property doesn't update properly with multiple dependencies

My property sum depends on 3 different properties (inside a model). I also have an other property that depends on sum. isZero and sum have a binding in a template {{isZero}} {{sum}}.

this is my simplified code:

sum: function () {
     console.log('sum is', this.get('p1') + this.get('p2') + this.get('p3'));
     return this.get('p1') + this.get('p2') + this.get('p3');
}.property('p1', 'p2', 'p3'),

p1: function () {
    console.log('update p1');
    return _data.p1;
}.property('data'),

p2: function () {
    console.log('update p2');
    return _data.p2;
}.property('data'),

p3: function () {
    console.log('update p3');
    return _data.p3;
}.property('data'),

data: {},

_data: {p1: 1, p2: 2, p3: 3},

isZero: function () {
    console.log('sum in isZero is', this.get('sum'));
    return this.get('sum') === 0;
}.property('sum'),

updateData: function () {
    this._data: {p1: 0, p2: 0, p3: 0};
    this.notifyPropertyChange('data');
} 

Some times, when I update data, sum and isZero are not correctly updated. If I call updateData(), these are the prints in console:

> update p1
> sum is 5
> sum in isZero is 5
> update p2
> update p3
> sum is 0

As you can see isZero is not updated anymore and sum updates twice. My template will display isZero false and sum 0. I know this code looks crazy but it is the closest I can show from my problem. I tried to reproduce in an ember jsbin, unfortunately it works correctly in there.

Anyone have any ideia why this is happening?

ps.: the updateData call is inside an ember run loop. I'm using ember version 1.9.1.