Mongodb – in standalone mode why does the mongo transaction is not rollbacking back

I’m looking for way to execute multiple statement in mongodb in a transaction. All or nothing kind of scenario. To make it easy to understand

trxTest> db.users.find({})
[ { _id: ObjectId("64ad882f451d9a5fc930eeb7"), name: 'werain' } ]
trxTest>

I’m looking to insert the multiple documents and one of the document conflict the uniquenss key on name ‘werain’

    client, err := mongo.Connect(context.Background(), options.Client().ApplyURI("mongodb://localhost:27017"))
    if err != nil {
        panic(err)
    }
    DB := client.Database("trxTest")
    coll1 := DB.Collection("users")

    session, err := client.StartSession()
    if err != nil {
        panic(err)
    }

    defer session.EndSession(context.Background())

    _, err = session.WithTransaction(context.Background(), func(ctx mongo.SessionContext) (interface{}, error) {
        _, err = coll1.InsertOne(context.Background(), bson.M{"name": "vikas"})
        if err != nil {
            return nil, err
        }

        fmt.Println("vikas is inserted")


        _, err = coll1.InsertOne(context.Background(), bson.M{"name": "sriram"})
        if err != nil {
            return nil, err
        }
        fmt.Println("sriram is inserted")

        fmt.Println("inserting is werain again")
        _, err = coll1.InsertOne(context.Background(), bson.M{"name": "werain"}) // This is where the transaction should failed
        if err != nil {
            return nil, err
        }
        fmt.Println("werain is inserted")
        return nil, nil
    }, nil)

   
    if err != nil {
        fmt.Println("got trx error", err)
    }

Here how the code looks
I was accepting the database would rollback my 2 entry for users with name "vikas" and "sriram"

but I see this

trxTest> db.users.find({})
[
  { _id: ObjectId("64ad882f451d9a5fc930eeb7"), name: 'werain' },
  { _id: ObjectId("64ad8c65f63f3f2b48f66169"), name: 'vikas' },
  { _id: ObjectId("64ad8c66f63f3f2b48f6616a"), name: 'sriram' }
]

Error

vikas is inserted
sriram is inserted
inserting is werain again

got error write exception: write errors: [E11000 duplicate key error collection: trxTest.users index: name_1 dup key: { name: "werain" }]

db version v6.0.1
Build Info: {
    "version": "6.0.1",
    "gitVersion": "32f0f9c88dc44a2c8073a5bd47cf779d4bfdee6b",
    "modules": [],
    "allocator": "system",
    "environment": {
        "distarch": "aarch64",
        "target_arch": "aarch64"
    }
}