As we see you have posted another post with ID: 667630 we have responded to you in that post but I would like to post our response in this post too:
It seems that there is an issue that causes the SelectedItem to not be updated when the SelectedIndex is changed when the control is initially Collapsed and vice versa. We logged this issue in our Public Issue Tracker System where you can track its status.
All the best,
the Telerik team