Unexpected undefined not assigned

import { enablePatches, immerable, produceWithPatches } from "immer";

const proto = { [immerable]: true, name: undefined };
const foo = Object.create(proto);

console.log(`[hasOwnProp] foo:`, Object.prototype.hasOwnProperty.call(foo, "name"));

enablePatches();
const [foo_next, patches, _] = produceWithPatches(foo, (x) => {
  console.log(`[immer] produce foo_next from immer`);
  x.name = undefined;
});

console.log(`[immer] foo_next patches:`, patches);

console.log(`[hasOwnProp] foo:`, Object.prototype.hasOwnProperty.call(foo, "name"));
console.log(`[hasOwnProp] foo_next:`, Object.prototype.hasOwnProperty.call(foo_next, "name"));

foo.name = undefined;
console.log(`[vanilla] assign name manually`);
console.log(`[hasOwnProp] foo:`, Object.prototype.hasOwnProperty.call(foo, "name"));

[hasOwnProp] foo: false
[immer] produce foo_next from immer
[immer] foo_next patches: []
[hasOwnProp] foo: false
[hasOwnProp] foo_next: false
[vanilla] assign name manually
[hasOwnProp] foo: true

[hasOwnProp] foo: false
[immer] produce foo_next from immer
[immer] foo_next patches: [
  {
    op: "add",
    path: [ "name" ],
    value: undefined,
  }
]
[hasOwnProp] foo: false
[hasOwnProp] foo_next: true
[vanilla] assign name manually
[hasOwnProp] foo: true