New Post: Cannot close SplitButton dropdown using IsOpen property.

I have a split button which has a dropdown that contains an ItemsControl that contains menu items, when a menu item is selected it calls a handler, the first thing I do is to attempt to close the dropdown using the following code.

private void OnClickOne(object sender, RoutedEventArgs e) {
  MyButton.IsOpen = false;
  ...
  // Do the work.
  ...
}

The dropdown does not disappear, I checked the value of IsOpen when it enters the handler and it is already set to false. The dropdown only seems to disappear if something else gains focus. Am I missing a style or setting somewhere? I was using an older version of the library but I downloaded 2.9.0 and it exhibits the same behaviour.

The following is an example of the XAML.
Any help would be greatly appreciated.

//*eggbox