As far as I am aware, the guaranteed seat thing is only for the main member so would not have helped here.

Receiving confirmation does not preclude a subsequent cancellation if there is found to be a problem with or cincern about the transaction (rightly or wrongly). There may have been an erroneous fraud marker for instance. Neither you nor I actually know what went wrong. And I don't suppose the autocancellation algorithm takes any notice of the make up of the group if it spots something about a transaction it doesn't like.

Like I say, calm and factual will get you further. You bave no proof of what went wrong in their back office