Using concat with an empty array on an array of subdocuments seems to incorrectly mark fields as changed

[m-weeks]

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

What is the current behavior?
Using concat on an already changed array seems to override the fact that the array has changed. This seems to be a very obscure issue and I have only been able to reproduce it by calling concat with an empty array.

If the current behavior is a bug, please provide the steps to reproduce.
Using the following model:

const MyChildSchema = new Schema({
  name: {
    type: "String"
  }
});

const MyModelSchema = new Schema({
  children: [MyChildSchema]
});

const MyModel = model("MyModel", MyModelSchema);

Doing the following operation:

var myModel = await new MyModel({
  children: [{ name: "John" }, { name: "Jane" }]
}).save();

var myModel = await MyModel.findById(newMyModel._id);

myModel.children = [{ name: "John" }];

myModel.children = myModel.children.concat([]); // In my case, this empty array comes from a function

myModel.children.push({ name: 'Mary' });

await myModel.save();

myModel = await MyModel.findById(newMyModel._id);
console.log(myModel.children)

The console.log should contain the names "John" and "Mary", but contains "John", "Jane", and "Mary". The reason is because mongoose performs only a $push query for "Mary"

What is the expected behavior?

The children on the document should only contain the names "John" and "Mary"

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Mongoose: 5.12.10
MongoDB: 4.2
NodeJS: 12.16.1

[IslandRhythms]

const mongoose = require('mongoose');
const {Schema} = mongoose;

const MyChildSchema = new Schema({
    name: {
      type: "String"
    }
  });
  
  const MyModelSchema = new Schema({
    children: [MyChildSchema]
  });
  
  const MyModel = mongoose.model("MyModel", MyModelSchema);

async function test() {
    await mongoose.connect('mongodb://localhost:27017/test', {
    useNewUrlParser: true,
    useUnifiedTopology: true
  });
  
  await mongoose.connection.dropDatabase();

  const myModel = await new MyModel({
    children: [{ name: "John" }, { name: "Jane" }]
  }).save();
  
  const search = await MyModel.findById({_id: myModel._id});
  
  search.children = [{ name: "John" }];
  
  search.children = search.children.concat([]); // In my case, this empty array comes from a function
  
  search.children.push({ name: 'Mary' });
  
  await search.save();
  
  const entry = await MyModel.findById({_id: myModel._id});
  console.log(entry.children)
}

test();