Rendered view problems vary with object location (skylight issue)

I have the situation where if i have a set of objects viewed in a viewport in Rendered mode, with skylight enabled and a directional light to simulate the sun, if the objects are near the origin, they render correctly, but if they are a long way from the origin, they do not. My guess is they are falling outside whatever sphere the skylight is using. it seems to affect polysurfaces and extrusions differently. the extrusions seem to work ok, but a polysurface the same shape exactly is rendered incorrectly - some faces are very dark. see screenshots attached below. first is shaded mode, second is rendered with model away from origin, third is with model at origin.


for reference, this is in Rhino for Mac V5.5.3 (5F88) running on a high end recent model iMac

bump. no-one from McNeel want to have a look at this?