Ok, i didn't know if these maps were important or not. They are used in the cars, but probably assigned to unused IDs.
Ignoring these materials and checking the scene again, i could find the problem: As said on my initial post, Forest creates a multi-sub with all material used in the Geometry List.
In this case, the number of unique materials exceeds 999 which is the limit for a multi-sub. Therefore, some IDs in the final geometry are wrong.
There are several workarounds (as create several FP objects with less materials), but the easiest method without changing anything in the scene, may be to use Forest Tools
just before rendering. If needed, you can revert to the original FP distribution with a couple of clicks.
I hope that helps.