ThreeJS Casting shadows through alpha masked mesh geometry three.js v0.170.0

HTML

<html>

    <head>

        <title> Live Example </title>

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

    </head>
    <body>

        <script type="module">
        import * as THREE from "https://cdnjs.cloudflare.com/ajax/libs/three.js/0.170.0/three.module.js"
        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.PlaneGeometry( 20, 20, 1, 1 )
        var bkMaterial = new THREE.MeshStandardMaterial( { color: 0x5080f0 } )        
        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.DirectionalLight(0xffffff, 1)
        light.castShadow = true
        light.shadow.radius = 10
        light.shadow.mapSize.width = 512
        light.shadow.mapSize.height = 512
        light.shadow.camera.near = 0.5
        light.shadow.camera.far = 20
        light.position.z = 10
        scene.add(light)
        const lightHelper = new THREE.DirectionalLightHelper( light, 1 )
        const lightShadowHelper = new THREE.CameraHelper( light.shadow.camera )
        //scene.add(lightHelper,...