Ember.js - filterBy() on custom transform attribute - strange behaviour

I have the following computed property

dailyAvailabilities: Ember.computed('availabilities@each.weekday', 'day', function() {
  let day = moment(this.get('day')).format('dddd');
  return this.get('availabilities').filterBy('weekday', day);
}),

the availability model looks like this

/models/availability.js

export default Model.extend({
  weekday: attr('weekday'),
  start: attr(),
  end: attr()
});

where weekday is a custom transform

/transforms/weekday.js

export default DS.Transform.extend({
  deserialize(serialized) {
    return moment().isoWeekday(serialized).format('dddd');
  },

  serialize(deserialized) {
    return moment(deserialized).format('d');
  }
});

Now when I go to a specific day, from a previous route I need to click once, go back and click again for the values to appear in the template. And if I reload the page all the values are gone. This doesn't happen if I filterBy() other model attributes like start or end. So it's clearly something wrong with the weekday but not sure what. Any directions on this are much appreciated.

Thanks in advance.