Ember changedAttributes() not show boolean change

I've been trying to store a list of changes to a model into another model before saving it, so that I can recall the changes to undo previous changes. This is the code that I'm using:

var changes = item.changedAttributes();
var newUndo = this.store.createRecord('undo', {
  item_id: item.get('id'),
  changes: changes
});

And my checkbox is a typical one:

{{input type="checkbox" checked=checked class="checkbox"}}

I have a component for the checkbox that sends a save action when checked changes, and the controller's save method is calling the code above when before saving the record:

import Ember from 'ember';
export default Ember.Component.extend({
  checked: false,
  model: null,
  watchChecked: function () {
    this.sendAction('save', this.get('model'))
  }.observes('checked')
});

This works well for string attributes, but with boolean checkboxes, it's not acting as expected. This is what I'm seeing:

Page load, unchecked item is checked:

changes: {}

Then unchecked:

changes: {print_copy_sent: true,true}

Then checked again:

changes: {print_copy_sent: false,false}