We always book cruises as true B2B -- even if they are offered as a longer cruise. The segments are also available book separately when we do this type of booking. We always received cruise credit and benefits for each segment. We always pay a deposit for each cruise and always get separate booking numbers, with b2b savings is posted later. We go through a TA, but have also done this on board. I have never had a problem in this type of booking. ( If one of the voyages is the milestone cruise, we must take the benefit then). For what it's worth, there is never any mention of Grand Voyage by either club ambassador or our TA. The only time we don't book this way is on ATW cruises. not sure if that helps.
In the case of that 17 day cruises, we would have booked a 10 day and 7 day separately -- gotten 2 credits and benefits. Not sure how people are getting shorted, maybe it's a recent change?