To reproduce:
this.radScheduler1.RightToLeft = System.Windows.Forms.RightToLeft.Yes;
this.radScheduler1.Appointments.Add(new Appointment(DateTime.Now, TimeSpan.FromHours(2), "آقای حقیقت"));
Please refer to the attached screenshot.
Workaround:
private void radScheduler1_AppointmentFormatting(object sender, SchedulerAppointmentEventArgs e)
{
e.AppointmentElement.UseHtml = false;
}