Check the video, bush disappears when display LOD4. (b_corylusHeterophylla_2s_summer.p3d)

https://youtu.be/VzJwbuZ8Zec

There is a case in LOD4 (angle of 60°) when all faces of boxes facing by normal to oposite direction of camera.
When you flip the marked box, this situation will never occur again.

Blue faces facing with normals to the camera and red away from the camera.