I confirm that the issue exists in our latest version. If this is an option for your scenario, you can use AppointmentMoving instead of the AppointmentDropping event.
Alternatively, you can get v2012.3.1017.20 from your Telerik account >> Manage Products >> Trial downloads >> RadControls for WinForms >> Download >> All product versions >> 2012.3. 1017.
I hope this helps.
All the best,
the Telerik team