version: 5.1.22 (Sorry I'm not migrated yet. It's fine if it works on v6)

Hello team, let me report an issue where I happend to face recently. My project code on GAE are written in Java basically but I start converting to Kotlin one by one since I like that language so much and it works on GAE well so far.
The following code worked fine and the server returns json data properly, it contains some data in an array attribute.
It's like:

@Entity
public class MyData extends Data { 
  private List<Record> records = new ArrayList<>();
   
  // snip constractor methods
  
  public static class Record: Serializable { ... }
}

{ 
  "records": [
    { "name": "foo" },
    { "name": "bar" }
  ]
}

However, after I convert that class to Kotlin, a response json data that it gets via objectify doesn't contain any values in an array.

@Entity
class MyData : Data() { 
  val records = mutableList<Record>()
  // snip
  class Record : Serializable {
    ...
  }
}

{ 
  "records": [  // <--- data is not saved!
  ]
}

I think I was missing some required configuration, so could you create a document about a case for Kotlin if possible?
Thanks,