To reproduce: follow the introduced approach in the following help article: http://www.telerik.com/help/winforms/scheduler-drag-and-drop-drag-and-drop-using-raddragdropservice.html 

When you maximize the form and return it to normal state, the appointment is rendered.

Workaround for Timeline view: after adding the dropped appointment you can refresh the scaling:

this.radSchedulerDemo.Appointments.Add(appointment);
this.radSchedulerDemo.GetTimelineView().ShowTimescale(Timescales.Days);