This is reproducible only if you set the RotationOrigin property to a value different than (0.5,0.5). Also, the rotation angle is wrong only on the first call of the Rotate() method of the service. 

To work this around you can create a custom RotationService and override its CalculateRotationAngle() method where you can calculate custom angle. 

http://docs.telerik.com/devtools/wpf/controls/raddiagram/features/services

https://github.com/telerik/xaml-sdk/tree/master/Diagram/CustomServices