That's definitely an option - when the "Non-Geometric Object" option is enabled, Forest creates instances as hidden nodes in the scene before rendering, and deletes them once rendering is completed (it's like an automatic Forest Tools, creating and deleting nodes in the scene before and after rendering). As mentioned previously, we will explore potential solutions in future versions. At the moment, in the case of the Arnold renderer, Forest items are converted to a mesh because the Arnold/3ds Max interface for Arnold doesn't support instancing scene nodes directly.
Best regards,