Issue with terrain assets floating/hovering after building

I'm having a weird issue where everything in my scene seems to be floating above the ground (as seen in Screenshot1), but this only seems to be happening after a build. Inside unity the preview is fine (as seen in Screenshot2). Has anyone seen this before? Or have any ideas on how to fix it?

Weirdly enough I have two levels doing this, and two that are completely fine. And I've built them all in exactly the same way.

 

Bump..... Any help would be greatly appreciated!

 

Check that your editor has same quality settings selected, as the build one?
(looks like some shadows are off in the 2nd image, and in first image has floating shadows problem?)

 

Just checked and both the build and editor have the same quality settings (max). The only difference was I was running the build in 2k res and the editor in my standard 4k res. I've just tried the build using 4k res though and it's doing the same thing.

I don't understand what would make the terrain assets float, or if they're not floating, then it must be the terrain itself that's been lowered somehow.

 

Update: I've just realised that it looks like all of the actual terrain has been flattened to the lowest point (the level of the path). Or Unity thinks it has (the stairs at the level start/end look correct other than the shadows coming off of them). But when mobs move around the map their shadows seem flat, like there is no gradient/slope on the path edges.

I've still got no idea what's going on though because it works perfectly in the editor (and on two of the four levels I've built).