I apologize for the misunderstanding. I created a simple test application and I was able to reproduce the problem. It is related to the fact that the IsDropDownOpen property changed event handler uses template parts that are not yet initialized if you set the property in XAML. Generally, it was not intended to be set this way, but I assume this behavior as a bug. It will be fixed in the next internal build, this friday.
Your points have been updated.
the Telerik team