Two days ago, I was not allowed to bring bottles of water onto the airplane, even though they were purchased inside security. This was in HKG. They had security on the jetbridge, after having handed over boarding passes immediately prior to the doorway. No metal detectors, just folding tables and security personnel rummaging through my stuff. Took out my bottles of water and pitched them.
The website for the airport specifically says that liquids purchased after security are OK to take on the plane (
here), so I'm not even sure how I would have known about this. I am currently a pregnant person, and I was really peeved about not having the water I wanted during my 13 hour flight. I was in J so they took care of me with plenty of water, happily.
My flight was direct to the USA, so maybe this is some odd interpretation of the liquid rules we're all familiar with from travel within the US.