I’am currently working on our persisters to update an answer option, I have the following code:

public async Task Update(ItemList list, Guid id, Option option, CancellationToken cancellationToken = default)
{
    var filter = Builders<ItemList>.Filter.And(
        Builders<ItemList>.Filter.Eq("_id", list.Id),
        Builders<ItemList>.Filter.Eq("Items._id", id)
    );
    var update = Builders<ItemList>.Update.Set("Items.$[i].Options.$[j]", option);
    
    var arrayFilters = new List<ArrayFilterDefinition<ItemList>>
    {
        new BsonDocumentArrayFilterDefinition<ItemList>(
            new BsonDocument("i._id", elementId)
        ),
        new BsonDocumentArrayFilterDefinition<ItemList>(
            new BsonDocument("j._id", option.Id)
        )
    };      
    
    var updateOptions = new UpdateOptions { ArrayFilters = arrayFilters };
    
    await _collection.UpdateOneAsync(filter, update, updateOptions, cancellationToken);        
}

In the database we store ids are guids structure looks like this:

{
  "_id": "ae39e133-18a6-43b0-8745-b13a00dd8632",
  “ItemId”: "af21eeb1-e952-4e10-af59-b13800cf8880",
  “Items”: [
    {
      "_t": “Item”,
      "_id": "e42f593e-1444-4f3a-9991-b15d008d9a99",
      "Options": [
        {
          "_id": "7dcccc2e-949a-4743-bd53-b15d008d9aa5",
          "Label": “test”,
          "Value": "a4e1c768-6cf9-417d-82bd-275c92be6a91",
          }
        }
      ]
    }
  ]
}

Putting the query in 3t studio it gives me MongoInvalidArgumentError: Update document requires atomic operators, this is a logged query that is being executed, c# driver does not give any error.

db.getCollection("Defintion").updateOne(
{ "update" : "Defintion", "ordered" : true, "$db" : "dbName", "lsid" : 
    { "id" : CSUUID("28fd5c50-4d0b-4a71-a2d4-d6c598b6f413") }, 
    "updates" : [{ "q" : { "_id" : "ae39e133-18a6-43b0-8745-b13a00dd8632", "Items._id" : "e42f593e-1444-4f3a-9991-b15d008d9a99" },
         "u" : { "$set" : { "Items.$[i].Options.$[j]" : 
             { "_id" : "7dcccc2e-949a-4743-bd53-b15d008d9aa5", "Label" : "Test2", "Value" : "a4e1c768-6cf9-417d-82bd-275c92be6a91"} } }, 
                 "arrayFilters" : [{ "i._id" : CSUUID("e42f593e-1444-4f3a-9991-b15d008d9a99") }, { "j._id" : CSUUID("7dcccc2e-949a-4743-bd53-b15d008d9aa5") }] }] }

)