const childSchema = new Schema({
    name: String
});
const Child = mongoose.model('Child', childSchema);

const parentSchema = new Schema({
    title: String,
    children: [{ type: Schema.Types.ObjectId, ref: 'Child' }]
});
const Parent = mongoose.model('Parent', parentSchema);

async function reproduce() {
    const parentDoc = new Parent({ title: 'My Parent', children: [] });

    const populatedDoc = await parentDoc.populate<{
        children: Types.DocumentArray<HydratedDocFromModel<typeof Child>>;
    }>('children');

    const childNameFromHydrated = populatedDoc.children[0]?.name; // OK

    const plainObject = populatedDoc.toObject();
    const childNameFromPlain = plainObject.children[0].name; // Property 'name' does not exist on type 'ObjectId'. ts(2339)
}