Hello,
Could you please try using the Corona 10 daily builds to see if the issue still persists? There were many bug fixes related to Forest Pack in Corona 10, so it’s possible that the issue has been resolved.
You can download Corona 10 daily build from here: https://forum.corona-renderer.com/ Chaos Corona 10 for 3ds Max - Daily Builds Changelog
Note: The daily builds are the newest, experimental versions of Corona Renderer intended mainly for testing purposes and to showcase the recently added features. They contain the latest bug fixes but are not guaranteed to be stable, and in some cases can even damage scene files.
Check this link to know more about it: https://support.chaos.com/hc/en-us/articles/4528206493329-Corona-for-3ds-Max-Daily-Builds-