Updating nested discriminators by dot notation

[nhitchins]

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

What is the current behavior?
Updating a nested discriminator by dot notation drops any attributes not defined in base schema even if discriminator key is provided in the update.

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

const shapeSchema = Schema({ name: String }, { discriminatorKey: 'kind' });
const schema = Schema({ shape: shapeSchema });

schema.path('shape').discriminator('Circle', Schema({ radius: String, color: String }));
schema.path('shape').discriminator('Square', Schema({ side: Number, color: String }));

const MyModel = mongoose.model('ShapeTest', schema);

doc = await MyModel.create({
  shape: {
    kind: 'Circle',
    name: 'circle test',
    radius: 5,
    color: 'red'
  }
});
 
const updatedDoc = await MyModel.findByIdAndUpdate(
  doc._id,
  {
      $set: {
         'shape.kind': 'Circle',
         'shape.name': 'updated name',
         'shape.radius': 10
      }
   }
);

updatedDoc.shape.name; // updated name
updatedDoc.shape.radius; // 5

What is the expected behavior?
The update will cast, (validate when runValidators=true) and update 'shape.kind' and 'shape.name' but drops 'shape.radius' which is defined in the discriminator schema. When updates use dot notation and path is a nested (or array) discriminator then should use discriminator key provided in the updates (or fallback to existing doc discriminatorKey value?) for correct casting. Currently the only way to set 'shape.radius' is to set strict=false (which defeats the purpose of defining a schema) or first retrieve the doc, apply the partial updates and .save(). Atomic operations are required.

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
Node.js 10.16.3
Mongoose 5.7.12
MongoDB 4.0.3