Well, it was easy to get rid of one possible cause of the problem: I used Boot Camp and installed Windows 10. Windows 10 reports battery life similar to that of macOS 10.12. And the battery life drops at about the same rate as macOS 10.12 if you're doing roughly the same things (playing a movie while browsing web pages in Safari/Edge). So it it's a but in macOS then precisely the same bug occurs in Windows 10.
However ...
When I started my tests (one in each OS) the predictors said that the battery would last another 3 hours 20 minutes. I ran my tests for 30 minutes. And after 30 minutes the predictors said that the batter would last another 3 hours and 10 minutes (not quite to the minute but close enough). So in both OSes the predicted battery life dropped 10 minutes in 30 minutes of use.
This suggests to me that the problem is not with low battery capacity, but with the prediction system. The prediction system uses the charge remaining in the battery, and the current drain rate, to predict how much longer the computer can function. The problem is this: the figure for the charge remaining the battery is not digitally produced by the battery. Instead it is a calculation based on the PD across the battery and graded on a curve. The curve has to be custom-created by the manufacturer based on their knowledge of how the battery preforms in real life. The curve is stored in firmware.
The computer uses the same result you do for its judgement. If it thinks that the battery will last less than another (for example) five minutes it will put the computer to hibernate mode. So if the curve is faulty, so is the computer's judgement about when the computer should hibernate.
My guess, based on the above, is that the battery does have something like whatever capacity Apple claims it does; the battery's power/charge curve, or some other part of power assessment, is incorrect in firmware; and that Apple will release a user-installable fix correcting this rather than repairing or replacing these computers.