Hmm, my first year flying CX in 2015, I didn't use my middle name on any CX ticket, although on my passport, (which I regret doing in my youth). I never had any issues.
About a year later, I was going to apply for e-channel and had "heard" that the e-channel officer wants your boarding pass to match your passport with middle name. When I booked the ticket, I added middle name, but it did not match my Asia Miles/MPC account name, event though the MPC number was on the ticket. I was going to use an upgrade award and there was a mismatch. So I had to change my MPC account name to include the middle name.
Turns out the e-channel officer didn't check my boarding pass. I used my MPC GO card as qualification for e-channel which did have my middle name at that point. So i was never questioned. But I'm not sure I needed to go through all that trouble of matching my account name or boarding pass to my passport to include my middle name.
At this point I have accepted that since CX now knows me with a middle name, I need to include it everywhere. I wish I had never put it on my passport 30 years ago.