Getting the terrain to interact with the cars?

When I edit the terrain the cars wont interact with it

For example ill extrude a part of the terrain upward like a sharp pyramid, when I drive the car into it it drives through it with no collision, why is that??

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here is an example

the car drives right through this box

 

I get this also. To fix it, I just closed the editor, then it collided with no problem. If worst comes to worst, just save the level and restart the game.

 

its a bug? i'll give that a try hope there's an update soon