When an appointment has a very short duration (seconds-level, e.g., 1 second and 0 minutes), it does not appear in the RadPrintPreviewDialog. The appointment is visible in the scheduler UI but missing from the print preview.

For example:

Dim ap1 As New Appointment
ap1.Start = New Date(Date.Today.Year, Date.Today.Month, Date.Today.Day, 15, 0, 0)
ap1.End = New Date(Date.Today.Year, Date.Today.Month, Date.Today.Day, 15, 0, 1)
ap1.Description = "not printed"
ap1.Summary = "not printed"
RadScheduler1.Appointments.Add(ap1)