Surface behind window glass becomes invisible

I have built several sliding windows using Vray ‘Glass Window Neutral’.

I have a wood material on the window frame. The window frame is (obviously) touching the glass and renders without issue, however if the frame touches the wall behind it, the section of the wall visible through the window becomes invisible. The next distant wall renders just fine. In effect the window creates a hole in the wall the frame is touching - the actual glass does not need to touch the wall, just the frame for this to happen.

If I move the window frame away from the wall, it then renders correctly with the wall visible through the window.

I have tried just about all possible adjustments, without luck. Is there some setting that I have wrong?