O fluxo de mudança do MongoDB 4.4.4 retorna todas as atualizações do documento em vez de uma condição filtrada correspondente?

var options = new ChangeStreamOptions
{
    FullDocument = ChangeStreamFullDocumentOption.UpdateLookup,
    BatchSize = 1
};

var filter = Builders<ChangeStreamDocument<UserInfo>>
    .Filter.Where(x =>
        x.OperationType == ChangeStreamOperationType.Update &&
        x.FullDocument.UserName.Contains("Alice"));

filter &= Builders<ChangeStreamDocument<UserInfo>>.Filter.Exists("updateDescription.updatedFields.Password");

var pipeline = new IPipelineStageDefinition[]
{
    PipelineStageDefinitionBuilder.Match(filter)
};

using (var cursor = await collection.WatchAsync<ChangeStreamDocument<UserInfo>>(pipeline, options))
{
    while (await cursor.MoveNextAsync())
    {
        foreach (var info in cursor.Current)
        {
            Console.WriteLine("Updated: " + info.FullDocument.UserName);
        }
    }
}

var watcher = DB.Watcher<UserInfo>("on-alice-updates-password");

watcher.Start(
    eventTypes: EventType.Updated,
    filter: b => b.Where(x => x.FullDocument.UserName == "Alice") &
                 b.Exists("updateDescription.updatedFields.Password"));

watcher.OnChanges += docs =>
{
    foreach (var doc in docs)
        Console.WriteLine("Updated: " + doc.UserName);
};