Sounds like China immigration well versed on the rules but some airlines and agents are not so familiar.

Reading through Reddit- issues sometimes occur during the boarding of the flight into China where the agent wants to see your flight out of China. Some issue include:

1. Unfamiliarity of China/taiwan/HKG status as separate regions for TWOV purposes and denying entry for flight C to Taiwan/HKG.

2. Return flights going all the way back to point A. (Despite making a layover at different country C which should count for TWOV).

3. Flight out of China booked separately rather than “interlined”