It's not worth worrying about since it got fixed, but the distinct likelihood is that when this was originally booked, it was not properly or timely ticketed. Sooner of later SQ finds itself with a reservation but no payment (that's what a ticket, even an award ticket, is) and cancelled.

Anytime you make reservations, you should always confirm that you hold, not merely a reservation, but an e-ticket number for each segment of your itinerary. This particularly true when your ticketing and operating carriers are not the same and it is even more true on awards where there is the risk that your reservation on SQ is cancelled and there is no award space left and UA can't get it back.