Recently I flew on a CX AM redemption but added my BA number for status. I was able to select seats for free but the BA number would never stick on the CX website, it always showed the Asia Miles number even when it came to check-in and on the mobile BP.
However, at airport check-in I didn't even go to a priority desk and the agent stuck priority tags plus my BA number showed on the paper BP.
It seems to me that the redeemed tix always stick with the AM account which was used for redemption. Actually you may add multiple FFP# to each reservation. That was why your status was recognized by the agent. But the main FFP# which would always be the AM acc which the miles deducted from