Component property is not changed inside mixin observer

I want to change the value of two properties of an Ember Component inside a Mixin it extends.

MyMixin = Ember.Mixin.create
  init: ->
    @set 'threshold', undefined
    @set 'oldItems', undefined
    @_super(arguments...)

    itemsLengthObserver: Ember.observer('items.content.length', ->
      actualItems = @get('items.content.length') or 0
      threshold = @get('threshold')
      oldItems = @get('oldItems')
      if actualItems > 0 and not threshold
        @set 'oldItems', actualItems
        @set 'threshold', true
      else
        ....

If I breakpoint after the @set threshold line and check the values for oldItems and threshold I can see the values were properly changed, however next time the items.content.length changes, values appear to be reset to default (both undefined)