I know HKG has two border control checkpoints on arrival, I think it's either West vs East or North vs South, and hence two enrollment centres.
Anyways, if my memory services, I think after certain hours (late at night), they close one and keep the other open. I found myself in this situation once where I entered through the hall that has a closed enrollment centre, so after I cleared immigration I walked over to the other hall and to my surprise, the office was open.

But this was 10pm. Not 11pm.