Immutable not working when deleting nested property by assigning new value to parent

[DenBo]

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

Bug

What is the current behavior?

When setting schemas to strict:'throw' and deleting a nested property on refetched document by assigning new value to parent, when document is revalidated, no error is thrown

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

describe('Immutable validator tests', () => {
  let Test = null

  before(() => {
    const SubSchema = new Schema({
      nestedProp: {
        type: String,
        immutable: true
      }
    }, {
      strict: 'throw'
    })

    const TestSchema = new Schema({
      object: {
        type: SubSchema
      }
    }, {
      strict: 'throw'
    })

    mongoose.model('Test', TestSchema)

    Test = mongoose.model('Test')
  })

  it('should produce an error, field in subschema is deleted when updating document', async () => {
    const testDoc = new Test({
      object: {
        nestedProp: 'A'
      }
    })

    await testDoc.save()

    const testDocRefetched = await Test.findById(testDoc._id)

    testDocRefetched.object = {}
    // works:  testDocRefetched.object.nestedProp = undefined

    await testDocRefetched.save().should.be.rejectedWith(
      'Test validation failed: object.nestedProp: Path `nestedProp` is immutable and strict mode is set to throw., object: Validation failed: nestedProp: Path `nestedProp` is immutable and strict mode is set to throw.'
    )
  })
})

What is the expected behavior?

Error should be thrown

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

mongoose: 5.9.20
node: 12.17.0
mongodb: 3.6.17