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Why is cardboard app displaying very incorrectly?

As a Unity newbie, I'm trying to export a very simple Vuforia project from Unity to Android Generic Cardboard, or Cardboard V1 and finding the orientation and scaling of the normal split screen display completely wrong. (photo attached)

Can anyone give any suggestions what might be causing it? All other VR apps run fine on my phone, including of type 'AR' apps, that have likely also been created with Vuforia in Unity, same as mine.

To let you know what I've briefly done: downloaded latest Unity, obtained the 'Vuforia AR Starter' asset (teapot on stones marker) and set ARCamera's digital eyewear script with Vuforia's recommended settings:

Eyewear Type = Video See-Through Stereo Camera Config = Vuforia Viewer Type = Generic Cardboard (Vuforia) or Cardboard v1 (Google)

In terms of the result for those two 'Viewer Type' settings, you'll see the two results in the attached screenshot.

My phone is  a oneplusone, Android version 6.0.1 (api 23).  Many thanks!