This seems like a problem one shouldn’t normally encounter, and I can’t imagine every Unity game with terrain and trees (or other objects) facing the same challenge. I’ve only used terrain with objects once maybe 4 years ago and did not have any shadow issues.
I was thinking there must be some way to extend the culling behaviour to some area around the camera, or flag the objects in some way. But I could only find this post, maybe that points in the right direction: