prisma: MongoDB: @unique constraint not working

Bug description

How to reproduce

Create a schema with @unique and add two objects with non-unique values.

Expected behavior

Throw an error

Prisma information

Environment & setup

• OS: Mac OS
• Database: MongoDB (using Azure CosmosDB with Mongo API)
• Node.js version: 15.11.0
• Prisma version: 2.21.2

prisma               : 2.21.2
@prisma/client       : 2.21.2
Current platform     : darwin
Query Engine         : query-engine e421996c87d5f3c8f7eeadd502d4ad402c89464d (at node_modules/@prisma/engines/query-engine-darwin)
Migration Engine     : migration-engine-cli e421996c87d5f3c8f7eeadd502d4ad402c89464d (at node_modules/@prisma/engines/migration-engine-darwin)
Introspection Engine : introspection-core e421996c87d5f3c8f7eeadd502d4ad402c89464d (at node_modules/@prisma/engines/introspection-engine-darwin)
Format Binary        : prisma-fmt e421996c87d5f3c8f7eeadd502d4ad402c89464d (at node_modules/@prisma/engines/prisma-fmt-darwin)
Default Engines Hash : e421996c87d5f3c8f7eeadd502d4ad402c89464d
Studio               : 0.371.0
Preview Features     : mongodb

Schema & code

// This is your Prisma schema file,
// learn more about it in the docs: https://pris.ly/d/prisma-schema

datasource db {
  provider = "mongodb"
  url      = env("DATABASE_URL")
}

generator client {
  provider        = "prisma-client-js"
  previewFeatures = ["mongodb"]
}

model User {
  id        String   @id @default(dbgenerated()) @map("_id") @db.ObjectId
  createdAt DateTime @default(now())
  updatedAt DateTime @updatedAt
  email     String   @unique
}

import { PrismaClient } from "@prisma/client";
const prisma = new PrismaClient();

async function main() {
  await prisma.$connect();
  await prisma.user.create({
    data: {
      email: "test@example.com",
    },
  });
  await prisma.user.create({
    data: {
      email: "test@example.com",
    },
  });
}

main()
  .catch(console.error)
  .finally(() => prisma.$disconnect());

Result

➜  prismamongo npx ts-node index.ts
[
  {
    id: '6081b778001e4db9009f1569',
    createdAt: 2021-04-22T17:50:48.444Z,
    updatedAt: 2021-04-22T17:50:48.445Z,
    email: 'test@example.com'
  },
  {
    id: '6081b779001e4db9009f156a',
    createdAt: 2021-04-22T17:50:49.237Z,
    updatedAt: 2021-04-22T17:50:49.238Z,
    email: 'test@example.com'
  }
]