I see how that could happen, but they should have reached out to all ticket holders who bought non-existent tickets.
I see an old TicketMaster announcement in my email saying that Carrie Underwood went on sale on 8/17/18. I don't think they had even formally announced the whole "Legendary Transformation" thing, much less announced that they were going to revamp the seats. So, theoretically, for a time, they probably sold seats on the old map. By Christmas, they should have at least been sending out warnings that "seat locations will be similar to what you buy, but exact seat numbers may change".
It could have been a secondary market issue too. That's one of the minor risks of buying on StubHub - you don't get any direct emails from the Garden when your concert gets cancelled or rescheduled, or if the seats change. The Garden might have told the primary buyers, but they just failed to notify StubHub (and the secondary buyers) that the seats had changed.