Document filters with an array of UUID

I have two fields I am trying to use to build a document filter (in App Services > Rules) for the Item collection.

One is a UUID (the _id of the Item document). The other is an array of UUID stored on the users profile.

This is the filter in its current form:

{
 "collaborator_id": { 
    "$in":  "%%user.custom_data.collaborators"
  }
}

It saves fine, but when I try to run the app and sync, it gives me an error “$in needs an array”:

ending session with error: error bootstrapping new query: error querying state store: error while querying the state collection "state_Item": (BadValue) $in needs an array (ProtocolErrorCode=201)

SCHEMAS:
Item:

{
  "title": "Item",
  "type": "object",
  "required": [
    "_id",
    "isComplete",
    "owner_id",
    "partnership_id",
    "summary"
  ],
  "properties": {
    "_id": {
      "bsonType": "uuid"
    },
    "isComplete": {
      "bsonType": "bool"
    },
    "owner_id": {
      "bsonType": "string"
    },
    "partnership_id": {
      "bsonType": "uuid"
    },
    "summary": {
      "bsonType": "string"
    }
  }
}

User:

{
  "title": "User",
  "properties": {
    "_id": {
      "bsonType": "objectId"
    },
    "partnerships": {
      "bsonType": "array",
      "items": {
        "bsonType": "uuid"
      }
    }
  }
}

EXAMPLES:
Here is a raw example of Item:

{
  "_id":{
    "$binary":{
      "base64":"TJiWdUgvTWOrNhZOCWhj6g==",
      "subType":"04"
    }
  },
  "isComplete":false,
  "owner_id":"652c671990aad0d369d94434",
  "collaborator_id":{
    "$binary":{
      "base64":"kYpeWOOlQ0iox5macnFfCw==",
      "subType":"04"
    }
  },
  "summary":"Ffff"
}

Here is a raw example of User (what is used for the user custom data):

{
  "_id":{
    "$oid":"652ddf6afda42fb20153d562"
  },
  "partnerships":[
    {
      "$binary":{
        "base64":"kYpeWOOlQ0iox5macnFfCw==",
        "subType":"04"
      }
    }
  ]
}

Hi @Paul_Newell and welcome to MongoDB community forums!!

Apologies for writing back so late. I hope you have figured the issue but if not, please find the details below.

As mentioned in the documentation Filters in App Services, the filters are applied to all requests except for the device sync requests.
The recommendation would be to use Document Filters which is required for Device Sync.

Let us know if the above filters works for you.
If not, could you help me with complete filter code you are using in your application.

Regards
Aasawari