will occur only if you set ResourcePopulationMode
| ResourcePopulationMode="ClientSide" />
However, in that case you won't be able to group by a resource type. Currently, this is a limitation of RadScheduler.
I also tested OnClientAppointmentInserting
and it occured as expected in my local tests. If you have a simple working sample that demonstrates the problem, you can open a support ticket and send it.
the Telerik team