Adding a new field to a Map on nested subdocument fails with updateOne

[codespork]

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

What is the current behavior?

ObjectExpectedError: Tried to set nested object field `children.$*` to primitive value `1` and strict mode is set to throw.
    at SubdocumentPath.cast (/repro/node_modules/mongoose/lib/schema/SubdocumentPath.js:157:11)
    at SubdocumentPath.SchemaType.applySetters (/repro/node_modules/mongoose/lib/schematype.js:1122:12)
    at Map.set (/repro/node_modules/mongoose/lib/types/map.js:90:11)
    at Map.$__runDeferred (/repro/node_modules/mongoose/lib/types/map.js:169:12)
    at new MongooseMap (/repro/node_modules/mongoose/lib/types/map.js:29:10)
    at Map.cast (/repro/node_modules/mongoose/lib/schema/map.js:59:12)
    at Map.SchemaType.applySetters (/repro/node_modules/mongoose/lib/schematype.js:1122:12)
    at Map.SchemaType._castForQuery (/repro/node_modules/mongoose/lib/schematype.js:1553:15)
    at Map.SchemaType.castForQuery (/repro/node_modules/mongoose/lib/schematype.js:1543:15)
    at Map.SchemaType.castForQueryWrapper (/repro/node_modules/mongoose/lib/schematype.js:1520:20) {
  path: 'children.$*'
}

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

const mongoose = require('mongoose');
(async () => {
  try {
    const parentSchema = new mongoose.Schema({
      data: new mongoose.Schema({
        children: {
          type: Map,
          of: new mongoose.Schema({ age: Number }, { _id: false })
        },
      }, { _id: false })
    });
    const Parent = mongoose.model('Parent', parentSchema);
    await mongoose.connect('mongodb://localhost:27017/test');
    const parent = new Parent({
      data: {
        children: {
          lisa: { age: 5 },
          john: { age: 7 }
        }
      }
    });
    await parent.save();
    await Parent.updateOne({ _id: parent.id }, { $set: { "data.children.kenny": { age: 1 } } });
    // this works
    // await mongoose.connection.getClient().db().collection("parents").updateOne({ _id: parent._id }, { $set: { "data.children.kenny": { age: 1 } } });
  }
  catch (error) {
    console.log(error);
  }
})();

What is the expected behavior?

{
  "data": {
    "children": {
      "lisa": { "age": 5 },
      "john": { "age": 7 },
      "kenny": { "age": 1 }
    }
  }
}

What are the versions of Node.js, Mongoose and MongoDB you are using? Note that "latest" is not a version.
nodejs = 14.15.1
mongoose = 6.0.5
mongodb (node driver) = 4.1.1
mongodb (database version) = 4.4.8