ThreeJS Casting shadows through alpha masked mesh

HTML

<html>

	<head>

		<title> Live Example </title>

		<style>
			body { margin: 0; position: fixed;}
			canvas { width: 100%; height: 100%; display: block;}
		</style>

		<script src="https://cdnjs.cloudflare.com/ajax/libs/three.js/110/three.min.js"></script>
	</head>
	<body>

		<script type="module">
    	import { OrbitControls } from "https://cdn.jsdelivr.net/npm/[email protected]/examples/jsm/controls/OrbitControls.js"

			var renderer = new THREE.WebGLRenderer( { antialias: true } );
			renderer.setSize( window.innerWidth, window.innerHeight );
			document.body.appendChild( renderer.domElement );
		  renderer.shadowMap.enabled = true;


			var scene = new THREE.Scene();
			scene.background = new THREE.Color( 0x80a0a0 );
			var camera = new THREE.PerspectiveCamera( 75, window.innerWidth/window.innerHeight, 0.1, 1000 );
			var controls = new OrbitControls( camera, renderer.domElement );

			camera.position.set( 5, -18, 15 );
			controls.target.set( 0, 0, 0 );

    

    var bkGeometry = new THREE.PlaneBufferGeometry( 20, 20, 1, 1 );
    var bkMaterial = new THREE.MeshStandardMaterial( { color: 0x008000 } )		
    var background = new THREE.Mesh(bkGeometry, bkMaterial)
    background.receiveShadow = true;
    background.position.z = -3;
    scene.add(background);
        
        
// Point light casting shadows      
    var light = new THREE.PointLight( 0xffffff , 1);
  	light.castShadow = true;            // default false
    light.shadow.radius = 10;
    light.position.z = 5;
    scene.add(light);
    var pointLightHelper = new THREE.PointLightHelper( light, 1 );
		scene.add( pointLightHelper );
      
      
    var tl = new THREE.TextureLoader();
		var alphaTex = tl.load( 'https://i.postimg.cc/nzTNj0yr/alpha-Mask.jpg');

      
		var planeGeometry = new THREE.PlaneBufferGeometry( 10, 10 );
 
      
    var planeMaterial = new THREE.MeshBasicMaterial( {
          side: THREE.DoubleSide,
          transparent: true,
           
          alphaMap: alphaTex,
        }...