Thanks, Bitsostring.
This morning, I found the Aercus was receiving a semi-specular solar reflection from the tiles, sufficient to cause the anomaly. Cure: we have a full-size bronze sculpture of a chimpanzee and I pulled it over the lit tiles, about half-way through the reflective area. The curve has restored itself to more-or-less normal.
Photo: Chimpanzee in place to block reflection from tiles, with Aercus unit about 1 m in front.
