Hi, we're investigating this issue, and seems related to the way that FP7 assigns the material to the render instances.
We use a method similar to VRayInstancer, which according to our tests, suffers the same problem. So we assume it's a bug or limitation in the V-Ray side.
We just reported it ChaosGroup. If the problem is fixed for VRayInstancer, hopefully that will solve FP7 as well.