Thanks for the tests, David... I never even considered that it might be the smoothing algorithm itself.
I'll report it to SM. In the meantime, I'll try different smoothing angles, see if it makes any difference.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)