@Override
    public float getHeightmapHeight(Vector2f xz) {
        // offset
        int halfSize = totalSize / 2;
        int x = Math.round((xz.x / getWorldScale().x) + halfSize);
        int z = Math.round((xz.y / getWorldScale().z) + halfSize);

        if (!isInside(x, z))
            return Float.NaN;
        return getHeightmapHeight(x, z);
    }
This will cause all normals for this terrain quad to be recalculated
jme3-terrain/src/main/java/com/jme3/terrain/geomipmap/TerrainQuad.java