How to Query Documents with an ObjectId Inside an Array Field in n8n's MongoDB Node

I’m experiencing difficulties using the MongoDB node in n8n to retrieve documents where a field (an array of ObjectIds) contains a specific ObjectId. My collection, pros, has documents with a campaigns field that looks like this:

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{
  "_id": ObjectId("64f5a1c2e4b0a8f7654a9cde"),
  "name": "John Doe",
  "campaigns": [
    ObjectId("67613823f92d1c8271bbafee"),
    ObjectId("1234567890abcdef12345678")
  ]
}

In MongoDB Compass, I can successfully query using:

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{ campaigns: ObjectId("67613823f92d1c8271bbafee") }

However, in n8n the query returns no results. For a single ObjectId field (non-array), I used this query on the forum which works:

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{ "$expr": { "$eq": ["$campaigns", { "$toObjectId": "{{ $json.campaignId }}" }] } }

When I try to query within an array, here are the tests I've attempted:

1. Using $in:

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{ "campaigns": { "$in": [ "{{ $json.campaignId }}" ] } }

Result: The MongoDB node returns an empty array:

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[{}]

2. Nesting $expr inside $in:

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{ 
  "campaigns": { 
    "$in": [ 
      { "$expr": { "$eq": ["$campaigns", { "$toObjectId": "{{ $json.campaignId }}" }] } }  
    ] 
  } 
}

Result:

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cannot nest $ under $in

My goal is to retrieve documents where the campaigns array contains the ObjectId that matches {{ $json.campaignId }}.

The workflow output when running the node is:

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[{}]

This issue occurs in n8n version 1.72.1 using SQLite via npm on n8n Cloud. Any suggestions on how to correctly construct the query for an array of ObjectIds would be greatly appreciated!

Thank you in advance for your help!

Guillaume.