As others have said, this is very much a glitch on AA.com.

I booked a OW award in J on CX SIN-HKG-JFK at the start of May and when I logged in to check, it showed "Economy (U)". I called AA and the representative told me that it was a glitch with the website related to Cathay bookings - as long as the booking code stays U, I'd be fine. Sure enough, two weeks later I got my boarding passes which were business the whole way.

Enjoy CX J - some complain that it's too narrow, but I find it very private with direct aisle access and it's truly flat which makes for a great sleep.