Oddly, the problem on the right wall was fixed when I noticed that some of the coordinates in the scene file didn't quite line up, leading to a slight "fold" in the face.
Correcting that in the scene file fixed the red wall but didn't explain:
other faces where the coordinates did line up and the normals were identical
why the renders on the official Minilight site don't show this "fold" given the coordinates in the original scene files also don't quite line up