Get Sight Line for VR User

I'm using Unity 5.1.2f and the "Virtual Reality Supported" setting, with a FPSController prefab for my player and an Oculus Rift DK2 with runtime/sdk 6.0.1.

I want to obtain a ray that represents the player's line of sight.

My code looks like:

 public GameObject camera; // linked to FirstPersonCharacter via Unity drag+drop
 void Update()
 {
     Debug.DrawRay(camera.transform.position, camera.transform.forward);
 }

The Debug ray is visible in the scene editor, but doesn't move when I look around with the headset. If I look around with the mouse (including vertically), the Debug ray follows the mouse movements. What's going on?

Workaround

If I enable mouse move smoothing on the FPSController, the drawn ray follows the headset as expected.