CoreMongooseArray#includes different behaviors in different versions

[koevas1226]

Do you want to request a feature or report a bug?
bug
What is the current behavior?
includes() different behaviors
If the current behavior is a bug, please provide the steps to reproduce.

const mongoose = require('mongoose');
mongoose.connect('mongodb://localhost:27017/test', { useNewUrlParser: true, useUnifiedTopology: true });
// model
const userModel = mongoose.model('User', {
  name: String,
  friends: [{ type: mongoose.SchemaTypes.ObjectId, ref: 'User' }]
});
// init function
async function init(){
  const user1 = new userModel({ name: 'user1' });
  const user2 = new userModel({ name: 'user2' });
  const user1Model = await user1.save();
  const user2Model = await user2.save();
  const user3 = new userModel({
    name: 'user3',
    friends: [user1Model._id, user2Model._id]
  });
  const user3Model = await user3.save();
}
// code
await init();
const user1 = await userModel.findOne({ name: 'user1' }).exec();
const user2 = await userModel.findOne({ name: 'user2' }).exec();
const user3 = await userModel.findOne({ name: 'user3' }).populate('friends', '_id').exec();
const ids = user3.friends.map(u=> u._id);
// behavior
// version 5.7.1 
ids.includes(user1._id); // true
ids.includes(user2._id); // true
// version 5.5.15
ids.includes(user1._id); // false
ids.includes(user2._id); // false

What is the expected behavior?
it should be the same behavior.
What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
nodejs v10.22.0
mongoose v5.5.15 && v5.7.1
mongodb v3.4.6

I had checked the release log but didn't found any breaking changes.
Since https://github.com/Automattic/mongoose/blob/master/History.md#554--2019-04-25 v5.5.4 already supported #includes(), so why it has different behaviors?

[vkarpov15]

I took a closer look and this is an unintended consequence of #7798 in v5.6.0: as of v5.6.0, user3.friends.map() returns a Mongoose array rather than a vanilla array, and Mongoose arrays support searching for documents by their _id. We intend to change this behavior in v6.0, see #8356. But changing it in a minor release seems a bit too heavy.

As a workaround, you can do const ids = [].concat(user3.friends.map(u=> u._id)); to ensure that you get a vanilla JavaScript array.