Actually it is not a bug. What is written for color and texture is correct and will work as can be seen in plagrounds 2 amd 4. Although having set the variable groundMaterial it makes more sense to use it and set up groundMaterial first and then assign it to ground.material
let groundMaterial = new BABYLON.StandardMaterial("Ground Material");
groundMaterial.diffuseColor = BABYLON.Color3.Red();
ground.material = groundMaterial;
Alternative
let groundMaterial = new BABYLON.StandardMaterial("Ground Material");
let groundTexture = new BABYLON.Texture(Assets.textures.checkerboard_basecolor_png.rootUrl, scene);
groundMaterial.diffuseTexture = groundTexture;
ground.material = groundMaterial;