Unexpected population of virtual with match() function in an embedded subdocument array about mongoose HOT 1 OPEN

const mongoose = require('mongoose');

const gradeSchema = new mongoose.Schema({
  studentId: mongoose.Types.ObjectId,
  classId: mongoose.Types.ObjectId,
  grade: String,
});

const Grade = mongoose.model('Grade', gradeSchema);

const studentSchema = new mongoose.Schema({
  name: String,
});

studentSchema.virtual('grade', {
  ref: Grade,
  localField: '_id',
  foreignField: 'studentId',
  match: (doc) => ({
    classId: doc._id,
  }),
  justOne: true,
});

const classSchema = new mongoose.Schema({
  name: String,
  students: [studentSchema],
});

const Class = mongoose.model('Class', classSchema);

async function run() {
  await mongoose.connect('mongodb://localhost:27017');
  await mongoose.connection.dropDatabase();
  const newClass = await Class.create({
    name: 'History',
    students: [{ name: 'Henry' }, { name: 'Robert' }],
  });
  
  const studentRobert = newClass.students.find(
    ({ name }) => name === 'Robert'
  );
  
  await Grade.insertMany([
    {
      studentId: studentRobert._id,
      classId: newClass._id,
      grade: 'B',
    },
  ]);
  
  const latestClass = await Class.findOne({ name: 'History' })
    .populate('students.grade')
    .select('-__v')
    .exec();
  
  // Robert's grade is populated in Henry's student object
  console.log(
    JSON.stringify(latestClass.toJSON({ virtuals: true }).students, null, 4)
  );
}

run();