When we load an event which is an instanceOfEventPHID, we can also load the original event and attach it. The we can do something like:

public function getIsCancelled() {
if ($this->getInstanceOfEventPHID() && $this->getOriginalEvent()->getIsCancelled()) { return true; }
return $this->isCancelled;
}

so as not to lose the cancellation state of an event when the parent recurrence is cancelled/reinstated.