To reproduce: 
- Add a drop down list and bind it to some data.
- Select a value from the drop-down and then type another value manually.
- Select other control and check the  SelectedValue/SelectedIndex.

Workaround:
int index = this.radDropDownList1.DropDownListElement.SelectedIndex;
object value = this.radDropDownList1.DropDownListElement.SelectedValue;