How did you get that from officials just enforcing the law on the books?
> The immigration officers explained to me that a new law that had come into effect in recent months meant I was unable to exercise the longer transit visa. My previous visits were not subject to that, I was told. The officials denied my request for details about this new law.
Most of the Chinese "law" are not lawfully legislated and are selectively enforced TBH.
Looks like some kind of special 规定 or 通知 targeted directly towards OP's visa.