This could happen if the default service code for a practitioner is set to the wrong service code. To set the correct default service code head to Setting -> Practitioner, Edit Practitioner, then select the correct default service and Save.
Also when editing the practitioner make sure that you have not got an appointment type that is associated with multiple service codes. An appointment type should only be used once within all your service codes. You can edit the Multi Types column. Or if you don't have multi types, to remove an appointment type you will need to remove the service and add it back in again.
If the default is correct or you have change the default you will need to force an appointment of service codes for appointments. To do this head to Settings -> Update appointments service codes, then select a date range, and click Update Appointments.