Validator called more than once (5.6.9), called never (5.6.11)

[qqilihq]

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

What is the current behavior?
This is a follow-up for #8067

Scenario: Validator is used on an object property. With 5.6.9 the validator gets called several times, with 5.6.11 it gets not called at all.

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

const mongoose = require('mongoose'); // no mongoose.version when using import?!

(async () => {
  const Model = mongoose.model('model', new mongoose.Schema({
    name: { type: String },
    level1: {
      level2: {
        type: Object,
        validate: {
          validator: (v: any) => {
            console.log('called validator with', v);
            return true;
          }
        }
      }
    }
  }));

  const doc = new Model({ name: 'bob' });
  console.log(mongoose.version);
  doc.level1 = { level2: { a: 'one', b: 'two', c: 'three' } };
  await doc.validate();
  console.log('done');
})().catch(err => console.error(err));

Running with Mongoose 5.6.9 I see:

$ ./node_modules/.bin/ts-node ./src/mongoose-8067.ts 
5.6.9
called validator with { a: 'one', b: 'two', c: 'three' }
called validator with one
called validator with two
called validator with three
done

Running with Mongoose 5.6.11 I see:

$ ./node_modules/.bin/ts-node ./src/mongoose-8067.ts 
5.6.11
done

I admittedly didn't try the version in between :-P

What is the expected behavior?
The validator should execute once with { a: 'one', b: 'two', c: 'three' }

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

[tho-masn]

I observed the same behaviour (see also here: #8113)
Looks like this misbehaviour got introduced in 5.6.10