Cloning schema with 'map' type attribute breaks validation when 'of' set to schema definition

[nhitchins]

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

What is the current behavior?
Cloning a schema Schema.prototype.clone() containing a 'map' type attribute with 'of' set to a new embedded schema breaks validation.
Note: a workaround is to create a new schema and use Schema.prototype.add() to first add the schema to be cloned and then add additional attributes - what is the difference between using baseSchema.clone() versus (new mongoose.Schema()).add(baseSchema)?

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

var mongoose = require('mongoose');

var mapSchema = new mongoose.Schema({
  text: String
});

var baseSchema = new mongoose.Schema({
  mapTest: {
    type: Map,
    of: mapSchema
  }
});

// Broken:
var extendedSchema = baseSchema.clone();
extendedSchema.add({ extendedValue: Number });

// Workaround:
// var extendedSchema = new mongoose.Schema();
// extendedSchema.add(baseSchema).add({ extendedValue: Number });

var BaseModel = mongoose.model('Base', baseSchema, "test");
var ExtendedModel = mongoose.model('Extended', extendedSchema, "test");

var data = {
  mapTest: {
    randomKey: {
      text: 'random-string'
    }
  },
  extendedValue: 2
};

// Save fails validation
var localModel = new ExtendedModel(data);
localModel.save(function (err) {
  if (err) return console.log(err);
});

What is the expected behavior?
Should correctly clone (or reference?) the embedded map type schema(s)

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node.js 10.16.0
Mongoose 5.7.11
MongoDB 4.0.3

[nhitchins]

Update: (new mongoose.Schema()).add(baseSchema) is obviously not a viable workaround because it does not add path discriminators defined on the baseSchema.

Missed this part in Schema.prototype.add() docs:

Adds key path / schema type pairs to this schema