doc.isModified returns false whenever multiple fields are specified

[ThoseGrapefruits]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the bug has not already been reported

Mongoose version

6.11.4, 7.4.1

Node.js version

20.5.0

MongoDB server version

6.0.2

Typescript version (if applicable)

No response

Description

Hello!

It seems like isModified had a regression somewhat recently and is now returning false whenever multiple paths are passed, even if one of those paths was modified. Not sure if I'm doing something wrong, but I was able to reproduce it on a fresh install of latest mongoose in an isolated repro script. I tested on the latest versions of 6.x and 7.x and the results are the same. I am upgrading from 6.9.0, where this repro script works as expected.

As far as I can tell in the documentation, this is still a supported feature.

Thanks for your time,
Logan

Steps to Reproduce

const mongoose = require('mongoose');

const Cat = mongoose.model('Cat', {
  name: String, 
  type: String
});

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

  await mongoose.connection.dropDatabase();

  const myCat = new Cat({ name: 'Lord Fluffles' })
  await myCat.save();

  myCat.name = 'Captain Fluffles';

  console.log('isModified multiple', myCat.isModified('name type'));
  console.log('isModified single', myCat.isModified('name'));
  console.log('isDirectModified', myCat.isDirectModified('name type'));
}

run().then(() => process.exit(0));

Expected Behavior

The repro script should print true on every line, but the isModified multiple line prints false.

[vkarpov15]

Fixed by #13674

[k-chop]

@vkarpov15 I also encountered this issue when I updated to 6.12.0.
Would it be possible for this fix to be backported to 6.x?

I've already fixed our app by passing array instead of string, but it would be nice if this is fixed in 6.x.
I opened backport PR: #13940