Since this might be a strange special case, there could be some obvious stuff I overlooked.
I'm using the Free script LPM (Layer Pass Manager) to render out some shots and passes, and one of these is a sun study, where I need to override the forest objects materials.
I assign the whole layer that includes all trees and FP objects to this Object-Pass using a simple greenish grey VRay material.
All trees that are not part of the FP distribution get overridden, but the FP trees aren't.
This is rather strange, and I really need to get this resolved ASAP.
Aside from that the FP trees render incredibly slow.
The project was originally started right after 2014 was released, and was my first 2014 max scene. Not sure of the FP version that existed back then, but I could render HD frames at about 10 minutes per frame.
Now with FP version 4.13 it slows to a crawl, and the statistic tool tells me it is discarding around 80-90% of my trees due to collision. This was not the case last time this was rendered. There have been no changes in the scene as far as units go.