The US has also sent the USS Ft. Worth, LCS-3,a nd a Burke destroyer into the area and they were tailed by a Chinese frigate.
The new bases the Chinese have created through reclamation efforts (there are seven of them and three of them have 3,000 meter runways), are impressive. But they are clearly artificial constructs on top of reefs that were mostly covered before at high tide.
See my photo album:
PRC South China sea Artificial Islands
Therefor, by international law they are not "islands," and not afforded a 12-mile limit.
I expect US vessels to sail up to possibly a four mile limit recognized in such instances. I expect the Chinese will bluster and say that they "wanre" us, and that we will announce that we maintained Freedom of Navigation. I do nto expect much more than that. sort of old cold war tactics but now in the South China Sea with China.