Trying to pass the runValidators to the mongoose findOneAndUpdate() method does produce great results (as of 6.0.5). Here is a generic solution to the problem. PS: If any mongoose / typescript Guru's out there know how to make this truly generic (where type T can be inferred and not assigned) please do let me know.

findOneAndUpdateWithValidation.ts

import { FilterQuery, UpdateQuery, QueryOptions, Model, model } from 'mongoose';

/**
 * As of mongoose 6.0.5 findOneAndUpdate does not handle the runValidators flag correctly.
 * If you are running upsert in combination with this flag then the update will not get validated correctly.
 * This is will fix that problem.
 * @param filter The query which will be used to find the document.
 * @param update The update which shall be applied.
 * @param options The optional Query Options to extend the behavior.
 */
function findOneAndUpdateWithValidation<T>(
  filter: FilterQuery<T>,
  update: UpdateQuery<T>,
  options?: QueryOptions
) {
  const model: Model<T> = this;
  const newDoc = new model(update);
  if (options && options.runValidators) {
    const error = newDoc.validateSync();
    if (error) throw error;
  }
  return model.findOneAndUpdate(filter, update, options);
}

test.ts

/**
 * @jest-environment node
 */
import 'jest-extended';
import * as mongoose from 'mongoose';
import { Database } from '../../../global';
import { findOneAndUpdateWithValidation } from './findOneAndUpdateWithValidation';

interface Person {
  firstName: string;
  lastName: string;
  age: number;
}

type PersonDoc = Person & mongoose.Document

interface PersonModel extends mongoose.Model<Person> {
  findOneAndUpdateWithValidation: typeof findOneAndUpdateWithValidation
}

const PersonSchema = new mongoose.Schema({
  firstName: { type: String, required: true },
  lastName: { type: String, required: true },
  age: { type: Number, required: true },
})

PersonSchema.statics = {
  findOneAndUpdateWithValidation,
}

const PersonModel = mongoose.model<PersonDoc, PersonModel>('person', PersonSchema);


describe('update validation tests', () => {
  // References mongoose.connect()
  new Database();

  // If this test passed then it demonstrates that run validators does not work when inserting.
  // Given that the age param is missing we expect an error, if you try however you will see the test passes.
  test('findOneAndUpdate - runValidators does not work on upsert', async () => {
    const rob = {
      firstName: 'Rob',
      lastName: 'Smith',
    }
    await expect(PersonModel.findOneAndUpdate({firstName: 'Rob'}, rob, {
      runValidators: true,
      upsert: true,
      new: true
    })).resolves.toMatchObject({
      firstName: 'Rob',
      lastName: 'Smith'
    })
  })

  // If this test passed then it demonstrates that run validators does not work when updating.
  // Given that the age param is missing we expect an error, if you try however you will see the test passes.
  test('findOneAndUpdate - runValidators does not work on edit', async () => {
    const rob = {
      firstName: 'Rob',
      lastName: 'Rogers',
    }
    await expect(PersonModel.findOneAndUpdate({firstName: 'Rob'}, rob, {
      runValidators: true,
      new: true
    })).resolves.toMatchObject({
      firstName: 'Rob',
      lastName: 'Rogers'
    })
  })

  // Test the custom method to see if validation works.
  // The request now correctly returns an error.
  test('findOneAndUpdateWithValidation - run Validators now gets processed', async () => {
    const jason = {
      firstName: 'Jason',
      lastName: 'Jones',
    }
    expect(async () => await PersonModel.findOneAndUpdateWithValidation<PersonDoc>({firstName: 'Jason'}, jason, {
      runValidators: true,
      upsert: true,
      new: true
    })).rejects.toThrow();
  })
})