To reproduce:

public Form1()
{
    InitializeComponent();
    this.radScheduler1.ActiveViewType = SchedulerViewType.Week;
    SchedulerWeekView weekView = this.radScheduler1.GetWeekView();
    weekView.RangeFactor = ScaleRange.QuarterHour;
    
    Appointment appointment = new Appointment(DateTime.Today.AddHours(23).AddMinutes(45), new TimeSpan(0,15,0), "Meeting");
    this.radScheduler1.Appointments.Add(appointment);
}

private void radCheckBox1_ToggleStateChanged(object sender, Telerik.WinControls.UI.StateChangedEventArgs args)
{
    this.radScheduler1.EnableExactTimeRendering = this.radCheckBox1.Checked;
}

Please refer to the attached gif file.

Workaround:
In order to deal with the border case with appointment ending at 00:00h, use a new TimeSpan(0,14,59)