It's probably a function call that was "improved" in the updated WDDM that came out with the Fall Creators Update. When I noticed that AA had stopped working I tried new NVIDIA video drivers, reinstalling DX9c, and a wide range of other control panel settings changes both in the game and in the NVCP all to no effect. What I finally found that actually worked was using NvidiaProfileInspector to set the AA Compatibility bits. In the past this was not needed, and the value could remain at the default 0x00000000. Now it needs one of several that end with 40, 41, or C1.

40
41
240
1040
1241
F0C1
100C1
20041
401241
401840
4412C1

There are subtle effect differences between each one, but generally they fall into the three main groups defined by the last two digits. Right now I've settled on the last one, as it's biased toward darkening the edges its aliasing.