Nested bucket aggregations not applied when using callback in bucketAggregation()

[shoxrux1996]

I'm trying to create a nested bucket aggregation using a callback in bucket(), but the nested aggregation never appears in the final DSL:

$regions = Enrollment::viaIndex()
    ->where('status', EnrollmentStatus::Studying)
    ->bucket(
        'regions',
        'terms',
        ['field' => 'student.region.id'],
        fn (Builder $builder) => $builder->bucket('student_count', 'terms', ['field' => 'student.id'])
    );

dd($regions->toSql());

Actual output:

"aggs": {
  "regions": {
    "terms": {
      "field": "student.region.id",
      "size": 1000
    }
  }
}

Expected output:

"aggs": {
  "regions": {
    "terms": { "field": "student.region.id" },
    "aggs": {
      "student_count": {
        "terms": { "field": "student.id" }
      }
    }
  }
}

Question
Is this the correct way to define nested bucket aggregations using a callback?
Or is there a different recommended approach for creating nested aggregations inside a bucket() call?
Thanks in advance!