Timestamps are lost when replacing a subdocument w/out any changes

[PenguinToast]

What is the current behavior?

When replacing an existing subdocument with timestamps enabled, if none of the fields within the subdocument change, then timestamps are lost/not set.

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

const mongoose = require('mongoose');

const assert = require('assert');

const childSchema = new mongoose.Schema({
    name: String
}, { timestamps: true, _id: false});

const testSchema = new mongoose.Schema({ child: childSchema, age: Number});

async function run() {
    await mongoose.connect('mongodb://localhost:27017');
    await mongoose.connection.dropDatabase();

    const Test = mongoose.model('Test', testSchema);

    const doc = await Test.create({ age: 10, child: { name: 'foo' } });
    assert(typeof doc.child.updatedAt !== "undefined"); // this works fine

    doc.child = { name: "foo" };
    await doc.save();

    assert(typeof doc.child.updatedAt !== "undefined"); // fails
}

run();

What is the expected behavior?

If timestamps are enabled on a subdocument, they should never be undefined after save'ing the document.

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

Tested this on 6.2.10. This worked as expected in 5.9.11, which we are upgrading from.

[PenguinToast]

Potentially related to #11603?

[PenguinToast]

Also, running doc.child.markModified("name"); before the save makes this test case pass, but doc.markModified("child.name"); does not.

[vkarpov15]

This was fixed in v6.3.1 with #11603. Please upgrade to >= 6.3.1 for the fix.