Good work Bob! That advances our understanding of the issue.

Quote - does only the first facet have to be triangulated, or the whole object?

I did a test of that. Triangulating the first face did not solve the problem, though it did change which face the wrong colour was inherited from. In my cube, the 'right' (first) face, which used to inherit from 'top' (second) face, now inherited from the 'bottom' (third) face. Neither did triangulating a lateral ring of four faces kill the bug. Triangulating the whole cube did work.