Ok, you assigned a VRayMtl to Forest001 (inner object), but it's not used by the outer FP because there is a material assigned in Forest001->Geometry->Material, and "Consolidate Materials" is enabled.
Specifically it has assigned the automaterial of Forest002, which is composed only by the viewport materials (FP Color ID...), because the grass blade (Cylinder001) had not any material assigned.
In FP5, it renders because the viewport materials are created on render, but in FP6 not. We had to change this behaviour, to fix an error with the interactive renders.
Please, try this:
- in Forest001->Geometry, click Material->Clear, to remove any material which is assigned.
- Repeat the process in Forest002.
- Assign some VRayMtl to Cylinder001.
Doing this, the material is propagated from the grass blade to all FP objects correctly.