[FIXED] Ground Plane printout no longer working in 8.6.7?

[EDIT: This issue appears to have been specific to a particular scene and is not replicated elsewhere.]

Vuforia version 8.6.7
Unity version 2019.1.9f

I have a scene with a Ground Plane Stage. I have been using the Emulator Ground Plane printout to test the ground plane in Unity Editor's play mode. It was working with Vuforia 8.1.x, but since updating to 8.6.7 it doesn't seem to work any more?

Is this a known issue? Am I missing something in the update that would make this work?

When I build to a device, it works as expected: I can detect ground planes and spawn the content.

Any help appreciated!