Outputting document with wrong _id when executing simplest query with strict = false

[upirr]

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

What is the current behavior?
When invoking simplest query findOne({ _id: }) returns correct document but it has wrong id from inner property
when setting strict = false.

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

Sample document

{
    "_id" : ObjectId("5a4cbff7c454930b8b90ff0f"),
    "test" : {
        "_id" : ObjectId("5a4cd761c454930b8b90ff54")
    }
}

const mongoose = require('mongoose');
mongoose.set('strict', false);

const Test= mongoose.model('Test', new mongoose.Schema({}, { collection: 'test' }));

(async () => {
	await mongoose.connect('mongodb://127.0.0.1:27017/test');
	const item = await Test.findOne({ _id: new mongoose.Types.ObjectId('5a4cbff7c454930b8b90ff0f') });
	console.info(item._id.toString()); // will return 5a4cd761c454930b8b90ff54
	process.exit();
})();

What is the expected behavior?
_id of returned document is '5a4cbff7c454930b8b90ff0f'

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node: v14.17.4,
"mongodb": "4.1.3",
"mongoose": "6.0.12"

[IslandRhythms]

This reporduction script will not work because _ids are created based on a timestamp. Furthermore, there are no documents in your model and therefore no _ids to find.

[upirr]

May be this will clear the picture. So, if there's a nested document ('someLegacyProp' in this case) that is not present in schema, then mongoose will replace root-level document id with that nested object id.

const mongoose = require('mongoose');
mongoose.set('strict', false);

const Test = mongoose.model('Test', new mongoose.Schema({}, { collection: 'test' }));

(async () => {
	await mongoose.connect('mongodb://127.0.0.1:27017/test');

	const unrelatedObjectId = new mongoose.Types.ObjectId();

	const { insertedId } = await mongoose.connection.collection('test').insertOne({
		someLegacyProp: {
			// some legacy property that is not in model but is de-facto present in doc
			_id: unrelatedObjectId,
			someOtherProp: true,
		},
	});

	const entity = await Test.findOne({ _id: insertedId });

	console.log(insertedId);
	console.log(unrelatedObjectId);
	console.log(entity._id); // entity._id will be equal to unrelatedObjectId

	process.exit();
})();

[IslandRhythms]

const mongoose = require('mongoose');
mongoose.set('strict', false);

const Test = mongoose.model('Test', new mongoose.Schema({}, { collection: 'test' }));

(async () => {
	await mongoose.connect('mongodb://127.0.0.1:27017/test');
	await mongoose.connection.dropDatabase();

	const unrelatedObjectId = new mongoose.Types.ObjectId();

	const { insertedId } = await mongoose.connection.collection('test').insertOne({
		someLegacyProp: {
			// some legacy property that is not in model but is de-facto present in doc
			_id: unrelatedObjectId,
			someOtherProp: true,
		},
	})

	console.log(await Test.find());
	const entity = await Test.findOne({ _id: insertedId });
	console.log(insertedId);
	console.log(unrelatedObjectId);
	console.log(entity._id); // entity._id will be equal to unrelatedObjectId


})();