unity_StereoEyeIndex broken in 5.6.0f3 or just not behaving as expected?

My situation:
VR-related custom shader. Various tweaks in the vert function to select from an over/under texture to display to each eye, various other behaviors. Shader logic in general is fine. I get what I expect from everything but the following:

This is always true (and obviously shouldn't be):

if (unity_StereoEyeIndex == 0) {
// Logic
}​

This works as expected:

if (unity_CameraProjection[0][2] < 0) {
// Logic
}​

Tested with GoogleVR SDK, both Daydream and Cardboard.

 

unity_StereoEyeIndex should only be effected in single-pass stereo mode, and as far as I know, only GearVR support that offically on mobile VR platform.

 

For sure won't work for GoogleVR or Cardboard, but should work with Daydream if Single pass stereo is enabled. It might only be working in the latest 5.6 / 2017 builds though.