Steps to reproduce.

1. Add a RadScheduler to a form, add some appointments.
2. Add grouping with at least two groups.
3. Drag an appointment from the second group (top to bottom) and drop it above the scheduler. The drag hint should be for invalid operation (striked red circle)
You will see that the appointment will disappear from the scheduler.

WORKAROUND
AddHandler Me.RadScheduler1.DragDropBehavior.Stopped, AddressOf DragDropBehavior_Stopped

Private Sub DragDropBehavior_Stopped(sender As Object, e As EventArgs)
    Dim element As TimelineGroupingByResourcesElement = Me.RadScheduler1.SchedulerElement.Children(0)

    For Each timelineViewElement In element.Children
        If timelineViewElement.GetType() = GetType(SchedulerTimelineViewElement) Then
            For Each applicationElement As RadElement In CType(timelineViewElement, SchedulerTimelineViewElement).Presenter.Children
                If applicationElement.GetType() = GetType(AppointmentElement) Then
                    applicationElement.Visibility = ElementVisibility.Visible
                End If
            Next
        End If
    Next
End Sub