Hi Marc,
Can you try removing the input to TessellationMultiplier in your material? That appears to scale the tessellation factor based on the distance from the vertex to the origin of the mesh. For small meshes (like the sphere in the material editor) that will give you a lot of tessellation. But for large meshes like the ground, 500 / that_distance will be a very small number and thus you will get no tessellation.
Since you have adaptive tessellation enabled you don’t really want to do your own distanced based multiplier, usually you would just want to use a constant. If you wanted to you should probably use Camera Position → Actor Position or Camera Position → World Position.