To reproduce:
1. Add a RadSheduler with one Appointment with more than one resources (e.g. add two EventIds in its  ResourceIds collection).
2.Group the scheduler by resources setting the GroupType property to GroupType.Resource.
3.Use SchedulerViewType.Timeline.

When you try to resize the appointment for one of the resources, the expected result is this appointment to be resized for all of its resources.

Workaround: Use the AppointmentMouseUp event and manually perform the refresh:

Private Sub sched_AppointmentMouseUp(sender As Object, e As SchedulerAppointmentMouseEventArgs)
    If e.Button = Windows.Forms.MouseButtons.Left Then
        Me.sched.SchedulerElement.Refresh()
    End If
End Sub