Mongodb aggregate cursor not returning array from lookup result - Golang

I have two MongoDB collections, one users and another for accesskeys. I want to get an aggregation of the users together with their accesskeys depending on whether they are subscribed to a specific channel or not. However when I run the code, the cursor data has the user data but the variable containing the lookup array is missing. The pipeline syntax is however working fine on mongodb-compass.

Data

Users Collection Sample

id: 42069
displayName: "tiredApe"
subscribedChannel: "BananaClips"

AccessKeys Collection Sample

userId: 42069"
apiKey: "yadayadayada"

Code

    pipeline := `[
    {
        '$match': {
        'subscribedChannel': 'BananaClips'
        }
    },
    {
        '$lookup': {
        'from': 'accesskeys', 
        'let': {
            'searchId': {
            '$toString': '$id'
            }
        }, 
        'pipeline': [
            {
            '$match': {
                '$expr': {
                '$eq': [
                    '$userId', '$$searchId'
                ]
                }
            }
            }
        ], 
        'as': 'accesskeys'
        }
    },
    {
        '$unwind': {
        'path': '$accesskeys'
        }
    }
    ]`

    var err error
    var cur *mongo.Cursor
    var doc bson.M
    opts := options.Aggregate()
    opts.SetAllowDiskUse(true)
    opts.SetBatchSize(15)
    if cur, err = userCollection.Aggregate(ctx, MongoPipeline(pipeline), opts); err != nil {
        panic(err)
    }

    defer cur.Close(ctx)
    for cur.Next(ctx) {
        fmt.Println(cur.Current.String())
    }

Helper function to convert the Pipeline string to a mongo.Pipeline object :

func MongoPipeline(str string) mongo.Pipeline {
    var pipeline = []bson.D{}
    str = strings.TrimSpace(str)
    if strings.Index(str, "[") != 0 {
        var doc bson.D
        bson.UnmarshalExtJSON([]byte(str), false, &doc)
        pipeline = append(pipeline, doc)
    } else {
        bson.UnmarshalExtJSON([]byte(str), false, &pipeline)
    }
    return pipeline
}

The expected result (as evidenced from the many trial on mongodb-compass) :

{
id: 42069,
displayName: "tiredApe",
subscribedChannel: "BananaClips",
accesskeys: {
userId: 42069,
apiKey: "yadayadayada"
}
}

The current result from Golang :

{
id: 42069,
displayName: "tiredApe",
subscribedChannel: "BananaClips"
}

Really looking for a breakthrough. Been stuck at it for half a day and can't figure out what I am missing or why the accesskey variable is missing from the result. Thanks in advance.