Mongoose schema default mutating original object nested properties provided to model constructor

[eriy]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.4.3

Node.js version

16.15.1

MongoDB server version

4

Description

Nested object are mutated in the original object that is provided as document for model constructor when in model schema defaults are declared and the original objects do not include values for this properties.

Steps to Reproduce

const mongoose = require('mongoose');
const Model = mongoose.model("TestModel", {
    topLevelField1: {
        nestedField1: String,
        nestedField2: {type: String, default: 'defValue'}
    },
    topLevelField2: {type: String, default: 'defValue'}
 });

 const originalData = {
    topLevelField1: {
        nestedField1: 'provided'
    }
 }
 const model = new Model(originalData);
 console.log(originalData)

Expected Behavior

Expected result (works in mongoose 5.7.0):
originalData === { topLevelField1: { nestedField1: 'provided' } }

Current result (mongoose 6.4.3):
originalData === { topLevelField1: { nestedField1: 'provided', nestedField2: 'defValue' } }

Expect that original object nested fields are not mutated by mongoose schema defaults.