Sorry for the late reply.
The object ID handling you mentioned is good for rendering masks together with the "normal" rendering.
The Render Mask script however, can render different kinds of masks very fast without having to render the whole beauty pass(es) again.
I think for the simplest kind of mask where you can render selected items in white while everything else is black (called luminance mask) the script internally assigns a white VrayLightMaterial to the selected objects and a black material to the rest.
In Forest Pack versions prior to 7 you would have to untick "consolidate material" for the script to work correctly. With Forest Pack 7 it always renders black on black.
Is there a setting in Forest to make the script work again? I think the author won't fix it. (In fact the last version is from 2018) Or can you fix it for us? I know that may people rely on this script to quickly render out a mask.