Why are my physics materials not where the layers are ... and how to fix?

I got a problem with my landscape material layers, the colors are where I have them painted, but the physics materials attached to the different layers are not at the same location.
Sometimes they are only a few meters away, sometimes they are wrong until reaching another landscape component…

Check this out: Landscape physics are wrong... - YouTube
The dust should only be at the sand, while on grass no dust should spawn.

I´m currently in 5.2, because 5.1´s landscape wasn´t working anymore (opening resulted in an engine crash) ^^