Road is collapsing when I play the level in the active level editor viewport

Hey guys,

I’ve got a bit of a weird issue that I can’t seem to work out. As you can see from the attached pictures in the design screen everything looks fine so far and works as it should but when I try to open the level in the active viewport mode the road below just collapses underneath. Any idea why this would be happening? I’ve checked the collisions and it appears to be fine. Can someone help me out?

Thanks in advance.

In the level designer - Imgur: The magic of the Internet
In the active viewport - Imgur: The magic of the Internet