I was learning terrain for myself and it had been working fine, but I tried launching the level and suddenly my ball character was clipping through most of the terrain. It had been working in editor but upon restarting my computer it no longer works there either. I used the Sculpt, Flatten, Smooth, and Ramp tools.
Here’s a gif of the problem: Why, UE4 Terrain? GIF | Gfycat
EDIT: By running the erosion tool over everything at a low strength (updating the terrain, I guess), it works in editor. I launched the project and ran into the same issue. Here’s a gif of the two running side by side: