NestJS converting mongodb snake case to camel case format but it returns undefined

I'm currently trying to create the model using nestjs mongoose schema, and i want to convert the mongodb data from snake case to camel case in my schema layer like this:

export type CharacterModelDocument = HydratedDocument<CharacterModel>;

@Schema({ timestamps: {
    createdAt: 'created_at',
    updatedAt: 'updated_at',
  },
})
export class CharacterModel implements CharacterModelInterface {
  _id: Types.ObjectId;

  @Prop({
    required: true,
    trim: true,
    index: true,
    alias: 'display_name',
  })
  displayName: string;

  @Prop({
    alias: 'profile_image',
  })
  profileImage: string;

  //another field
}

export const CharacterModelSchema =
  SchemaFactory.createForClass(CharacterModel);

CharacterModelSchema.plugin(snakeToCamel); //Installed Plugin

with my plugin like this:

import { Schema } from 'mongoose';

// function to convert snake_case to camelCase
const toCamel = (str: string): string => {
  return str.replace(/[-_][a-z]/gi, ($1) => {
    return $1.toUpperCase().replace('-', '').replace('_', '');
  });
};

const isObject = (obj: any): boolean => {
  return (
    obj === Object(obj) && !Array.isArray(obj) && typeof obj !== 'function'
  );
};

// transform key into the object in camel case
const transformKeys = (obj: any): any => {
  if (isObject(obj)) {
    const n: any = {};

    Object.keys(obj).forEach((k) => {
      n[toCamel(k)] = transformKeys(obj[k]);
    });
    return n;
  } else if (Array.isArray(obj)) {
    return obj.map((i) => {
      return transformKeys(i);
    });
  }
  return obj;
};

// snake to camel case plugin
export function snakeToCamel(schema: Schema) {
  const transform = (doc: any, ret: any) => {
    return transformKeys(ret);
  };

  schema.set('toJSON', { transform });
  schema.set('toObject', { transform });
}

And when i get the data, everything it works like normally

{
  Id: {
    buffer: {
      //objectid
    }
  },
  profileImage: 'profile.jpg' //camel case,
  //other fields
  createdAt: {},
  updatedAt: {},
  _V: 0,
  displayName: 'character name' //camel case
}

but when i try to access the value inside display name by using the converted camel case (e.g, displayName), it returns undefined where actually they had a value inside it.

so do you guys knows how to solve this?

Thank you