Type Definition of FlattenMap isn't working for non trivial types such as Date and ObjectId

[borfig]

Do you want to request a feature or report a bug?

bug

What is the current behavior?

The type definition of FlattenMaps makes it impossible to be used with ObjectId and Date fields (and maybe other non-trivial types?)

If the current behavior is a bug, please provide the steps to reproduce.

The following TypeScript fails to build:

import { Model, Schema, Types, model  } from "mongoose";
interface Foo {
  someDate: Date;
  someId: Types.ObjectId;
  someNumber: number;
  someString: string;
}
const fooSchema = new Schema<Foo, Model<Foo>>({
  someDate: { required: true, type: Date },
  someId: { required: true, type: Schema.Types.ObjectId },
  someNumber: { required: true, type: Number },
  someString: { required: true, type: String },
});
const fooModel = model("foos", fooSchema);
async function someLogic() {
  const items = await fooModel.create<Foo>([
    {
      someId: new Types.ObjectId(),
      someDate: new Date(),
      someNumber: 5,
      someString: "test",
    },
  ]);
  const json = items[0].toJSON();
  const someDate: Date = json.someDate; // Type 'FlattenMaps<Date>' is not assignable to type 'Date'
  const someId: Types.ObjectId = json.someId; // Type 'FlattenMaps<ObjectId>' is not assignable to type 'ObjectId'
  const someNumber: number = json.someNumber; // OK
  const someString: string = json.someString; // OK
  console.log(someDate);
  console.log(someId);
  console.log(someNumber);
  console.log(someString);
}

My tsconfig.json

{
  "compilerOptions": {
    "module": "commonjs",
    "target": "es2020",
    "lib": [
      "dom",
      "es2020",
    ],
    "esModuleInterop": true,
    "strict": true,
    "moduleResolution": "node",
    "resolveJsonModule": true,
    "sourceMap": true,
    "outDir": "dist",
    "baseUrl": ".",
    "jsx": "react"
  },
  "include": [
    "src/**/*.ts*",
    "src/locales/**/*.json"
  ]
}

What is the expected behavior?

FlattenMaps<T> needs to be exactly T where T is Date or Types.ObjectId

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.

Note.js: 4.18.2 (although I don't believe it is relevant to this bug)
Mongoose: 6.1.2 (I believe the issue exist since 6.0.12)
MongoDB: 4.4.10 (although I don't believe it is relevant to this bug)