MongoDB: aggregation $lookup between ObjectIds returning an empty array

Question

I'm trying to perform an aggregation using $lookup between two fields of type ObjectId. I've already checked my data and it should be returning something. Also, the schemas indeed define the fields as ObjectId, so if I am not mistaken I should not need to convert the data type as I've seen in some posts. Here's my query:

db.workerlocationcontexts.aggregate([ 
    {
        $lookup: {
            from: "LocationSensors",
            localField: "sensor",
            foreignField: "_id",
            as: "test"
        }
    }
])

Here are my collections/sample data:

• WorkerLocationContexts

  {
    "_id":{"$oid":"615676c885ccad55a493503b"},
    "updatedAt":{"$date":"2021-10-01T02:47:36.207Z"},
    "createdAt":{"$date":"2021-10-01T02:47:36.207Z"},
    "sensor":{"$oid":"6181e5f83fca98374cf901fd"},
    "worker":{"$oid":"6153dcfb58ad722c747eb42d"},
    "__v":0
  }
  

• LocationSensor

  {
    "_id":{"$oid":"6181e5f83fca98374cf901fd"},
    "name":"Location Sensor 1",
    "description":"Location sensor for Location 2",
    "location": "$oid":"6181df3b3fca98374cf901fb"},
    "trackerType":"RFID",
    "sensorType":"ENVIRONMENT",
    "type":"LOCATION",
    "__v":0
  }
  

The result I'm getting are all my WorkerLocationContexts, but the field "test" returns as an empty array.

Can someone help me?

Thanks in advance!