Stereo camera positions with shader

I'm using the WebXRCameraSet plugin, which sets up a left and right camera. When I compile to WebGL, I'm able to load the scene into my Oculus HMD, but the same image is rendered for each eye.

In the shader, I'm getting the camera position with:

float3 GetCameraPosition() { return _WorldSpaceCameraPos; }

When I query unity_StereoEyeIndex, it's always 0 in my shader. How might I properly connect this camera rig to my shader?