Shadow inside disappear in certain angle

the bottom mesh is ExtrudeGeometry with two materials.

const hiddenMaterial = new THREE.MeshBasicMaterial({ visible: false })
const glassWall = [
  new THREE.MeshLambertMaterial({
    color: basicColor,
    flatShading: true,
    side: THREE.DoubleSide,
    transparent: true,
    opacity: 0.6
const mesh = new THREE.Mesh(extrudeGeometry, glassWall)

it is overlapping with a mesh as ground

const ground = new THREE.Mesh(
  new THREE.PlaneGeometry(x, x),
  new THREE.ShadowMaterial({
    opacity: 0.6,
    polygonOffset: true,
    polygonOffsetFactor: 1,
    polygonOffsetUnits: 1

when rotating camera, the shadow inside is disappeared in certain angle.
i can confirm the distance to extrudeGeometry is irrelevant.
anyone knows why ???