Further to my earlier post, I have now managed to identify what is causing the specularity bug. RECAP: A figure with a single distant light, animated in a 360 degree circle, and each frame has wildly differing specularity. The problem is the shadow blur radius. If it is anything but zero, regardless of if you are are using depth mapped or ray traced shadows, creates this instability (see frames below). There was a shadow blur bug of another kind, which was supposedly fixed in version 12.0.396, but this is a new bug

WORKAROUND A distant spot does not have the specularity bug

Below - two consecutive frames - light not animated