Ember light-table sorting column

I would like to sort a table by column. This works as expected but the icons for ascending is displayed at every 3rd click.

This is my code (working with ember 2.14):

export default Ember.Component.extend({

  model: null,
  table: null,

  sortProps: ['lastName:asc'],
  sortedPeople: computed.sort('model', 'sortProps'),

  columns: computed(function() {
    return [{
      label: 'Vorname',
      valuePath: 'firstName',
    }, {
      label: 'Nachname',
      valuePath: 'lastName',
    }];
  }),

  table: computed('sortedPeople', function() {
    return new Table(this.get('columns'), this.get('sortedPeople'));
  }),

  actions: {
    onColumnClick(column){
      let dir = column.ascending ? ':asc' : ':desc';
      this.setProperties({
        sortProps: [ column.get('valuePath') + dir ]
    });
  }
)};