working for another thing in sogo
I tested your issue on SOGO3, it is workable for me, what I did :
Create an event on the calendar, add an attendee, save and the invitation is sent.
if the attendee accepts, then a notification is sent to the owner (by email)
If the owner of the event modify the date and save it, then the modification is sent to the attendee(by email).
If the attendee refuses once the invitation has been accepted, then an update with the renunciation is sent to the owner of the event(by email).
Does it sound what you searched.