I am encountering a similar problem with US Airways redemption - which i guess is using the same system as AA.

I am looking at 22 May HKG-MXP in F and have checked CX.com which shows 1 seat available but US agent cant see it at all. I have double check with BA.com, QF.com and made a call to 2747-5500 which all confirm the seat.

Furthermore, to gather more data point, I have asked for 21 May, where HKG-MXP is showing 2 seats on both CX.com and BA.com but US agent can only see 1.

So it seems US and AA has some problem / intentionally restricting redemption on partners, similar to what US did to LH inventory. It seems it is not CX's problem as the seat is available to BA and QF.