I've always had trouble with this feature, getting scales right, effect right, effect to hold over when a new surface layer is added between, etc, etc,
this, well, this is a new one: If the surface layer has smoothing activated, strange things happen. The surface layer has micro displacement for sand, and sand waves, at a tiny scale, that's it. So I assume this is because the layer smooths the terrain for the sand. Just thought I'd show off this quirk and make it known. Lol