To reproduce:  

1.Change the first day of week to Monday:

            SchedulerMonthView view = new SchedulerMonthView();
            CultureInfo ci = new CultureInfo("en-US");
 
            ci.DateTimeFormat.FirstDayOfWeek = DayOfWeek.Monday;
            view.CurrentCulture = ci;
            this.radScheduler1.ActiveView = view;
2. Create an appointment with a recurrence rule illustrated in the attached screenshot. Imagine that today is Wednesday and the recurrence rule starts on Monday from the same week.
As a result, you will notice that the WeeklyRecurrenceRule.FirstDayOfWeek is not set and the appointment occurs on the wrong Sundays. Refer to the attached screenshot. 

Workaround:

        private void radScheduler1_AppointmentAdded(object sender, AppointmentAddedEventArgs e)
        {
            if (e.Appointment.RecurrenceRule != null)
            {
                WeeklyRecurrenceRule r = e.Appointment.RecurrenceRule as WeeklyRecurrenceRule;
                r.FirstDayOfWeek = DayOfWeek.Monday;
            }
        }