[BUG] Strange behaviour with set function

[Zwimber]

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

What is the current behavior?

1. Modifying the object in setters behaves strangely
2. Fields created in set functions are not added to the object

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

const mongoose = require('mongoose')
const Schema = mongoose.Schema
const db = mongoose.createConnection()

const NameSchema = new Schema({

    full: {
        type: String,
        set: function (v, x, y) {

            // One way to set values
            this.set('first', 'XXX')

            // Another way
            this.last = 'YYY';

            // Something weird
            this.weird = 'ZZZ'

            // Log current value
            console.log('Current value', this) // { first: 'XXX', last: 'YYY' } // Logs this on each occasion

            return v + ' BBB';
        }
    },
    first: String,
    last: String,
    
}, { _id: false, strict: false })

const User = mongoose.model('user', new Schema({

    name: {
        type: NameSchema,
        default: {},
    }

}, { _id: false, strict: false }))

test('(0) User set name', done => {

    const s = new User()
    s.name = { full: 'AAA', other: 'ZZZ' }
    console.log(s) // { name: { full: 'AAA BBB', other: 'ZZZ' } }
    done()
})

test('(1) User set name', done => {
    const s = new User({ name: { full: 'AAA', other: 'ZZZ' } })
    console.log(s) // { first: 'XXX', last: 'YYY', full: 'AAA BBB', other } }
    done()
})

test('(2) User set name', done => {
    const s = new User()
    // s.name = {}
    s.name.full = 'AAA'
    s.name.other = 'ZZZ';
    console.log(s) // { first: 'XXX', last: 'YYY', full: 'AAA BBB' } }
    done()
})

Result in a table:

	name.full	name.first	name.last	name.weird	name.other
Test 0	AAA BBB	-	-	-	ZZZ
Test 1	AAA BBB	ZZZ	YYY	-	ZZZ
Test 2	AAA BBB	ZZZ	YYY	-	-

What is the expected behavior?

Firstly I expect test (0) to produce the same result as test (1) and test (2). Please note that everything works very differently if you comment out default: {} property (and uncomment //s.name = {}.

Secondly I'd expect that on each of the tests the property weird to exist.

Please mention your node.js, mongoose and MongoDB version.

Node: 11.10.0
Mongoose: 5.4.17
MongoDB: 3.6.9