Equation
Lieutenant General
US challenges any nation that claims it has sovereign control of water ways outside internationally recognized territorial limits. That usually means 12 miles for islands and coasts, 3 miles for land features, and 500 meters for artificial islands. You will note USN spokesman said USS Decatur stayed outside 12 mile limits of all Parcel island land features, and China's Defense Ministry didn't contradict that claim. Therefore we could safely assume it is true. Given that fact, it doesn't matter what "baseline" China established in 2016, 1996, 1066, or even 66 BC, those baseline don't impede freedom of the seas outside internationally recognized sovereign territorial limits.
Quite frankly, I think the US is making a mistake by not sailing (safely) near the 3 mile limit of land features and near 500 meter of artificial islands. It should do just that, if FON has any real meaning. But, that's just my personal opinion.
It doesn't matter what excuses the US makes for it's FON Ops the deeper reasoning behind is to try to deny China's ability to counter the US chances to embargo and surround the PRC at will whenever somebody step out of line of the gunboat policy. We can also safely assume that the US has stepped over someone else's 'baseline' for it's own purpose. The world doesn't revolve around whatever the US wishes. It's that simple, the world has changed and so is the time and politics.