NavMeshPath cutting through slopes in CalculatePath

My NavMeshPath doesn't stick to the surface of slopes (or stairs) but sometimes takes a shortcut and goes through objects even though the NavMeshSurface looks perfect. I already played around with values of my Agent and the NavMeshSurface without good result. I have found this issue in other forums too, but never with the right answer. Hopefully one of you can help.
In following example image you can see an extreme example (for bettr viz I added arrows along the path) where the line cuts through the second slope and goes directly to the targen in the upper level (top right corner).

 

Did you ever figure this out? I'm trying to get the NavMeshPath on a terrain to transfer elsewhere, but it won't work