Terrain collider failing in unity 5/17?

Maybe I'm a little too reliant on the inbuilt physics engine, but if a rigidbody is either traveling too fast into the collider or hits the collider at an awkward angle, the terrain collider lets it fall straight through. This has only been happening since unity 5 up to 2017, so I've still got an old installation of 4.3.2 that I can rely on which doesn't have this problem.

It seems like in unity 4, the terrain collider seemed like it covered from the surface to the bottom, but since unity 5 it only covers the surface like a mesh collider. This leads me to believe that they wouldn't see it as a 'bug'. I've looked around and I haven't seen anybody raise this issue anywhere so I'm not sure if it's me relying on inbuilt physics too much. Thanks for any suggestions or fixes, if any.