The properties set at design time are not saved

Hello, Steve,    

Following the provided gif file, I have prepared a sample project to test the behavior on my end with the latest version. Indeed, the Selected property value is not serialized at design time for RadDropDownList. 

I have logged it in our feedback portal by creating a public thread on your behalf. You can track its progress, subscribe for status changes and add your comments on the following link - feedback item.

I have also updated your Telerik points.

Currently, the possible solution that I can suggest is to set the Selected property at run time: 

https://docs.telerik.com/devtools/winforms/controls/dropdown-listcontrol-and-checkeddropdownlist/dropdownlist/features/selection#programmatically-select-items 

I hope this information helps. If you need any further assistance please don't hesitate to contact me. 

Regards,
Dess | Tech Support Engineer, Sr.
Progress Telerik