as you need to update LOD system of your buildings there should be graphics option for visibility range – or distance fog…
just becouse rendering such of objects maybe a little problem for most not top end pc
i tried to focus on your LOD while zooming in and out and for example objects like satelite TV antena in that count in big city … it needs to change whole LOD not just vanish some small objects, or at least desrease rendering distance for that small parts of objets
also it seams that game engine does render buildings even they are hiden behind hill…