async Task Main()
{
	#region initialise MDB
	BsonSerializer.TryRegisterSerializer(new ObjectSerializer(type => ObjectSerializer.AllAllowedTypes(type)));

	var clientSettings = MongoClientSettings.FromConnectionString("mongodb://localhost:27017/?retryWrites=true");
	
	clientSettings.LinqProvider = LinqProvider.V3; //<<<---- Change to V2 and the Projection Works
	clientSettings.ApplicationName = "Bug Demo";
	clientSettings.ReadEncoding = new System.Text.UTF8Encoding(false, false);

	var dbClient = new MongoClient(clientSettings);
	Console.WriteLine(dbClient.GetType().Assembly.FullName);
	Console.WriteLine($"LinqProvider: {clientSettings.LinqProvider}");
	Console.WriteLine("===================================");
	var db = dbClient.GetDatabase("bugDb");
	var clientAccountCollection = db.GetCollection<ClientAccountModel>("bankAccount");
	#endregion

	const string BSB = "111222";
	const string NUMBER = "0456789";
	const long INTERNAL_ACCOUNT = 9876543210L;

	#region Setup Test Data if collection is empty
	if (await clientAccountCollection.CountDocumentsAsync(FilterDefinition<ClientAccountModel>.Empty) == 0)
	{
		var clientAccountModel = new ClientAccountModel { 
															InternalAccountNumber = INTERNAL_ACCOUNT,
															Bank = new BankModel { 
																					Bsb = BSB, 
																					Number = NUMBER,
																				 } 
														};

		await clientAccountCollection.InsertOneAsync(clientAccountModel);
	}
	#endregion
	
	var filter = Builders<ClientAccountModel>.Filter.Eq(clientAccount => clientAccount.Bank.Number, NUMBER);
	var projection = Builders<ClientAccountModel>.Projection.Expression(clientAccount => clientAccount.Bank.Bsb);
	
	var result = await clientAccountCollection.Find(filter)
											  .Project(projection)
											  .FirstOrDefaultAsync();
	if(result == null)
		 Console.WriteLine("Result is NULL");
	else
		Console.WriteLine(result == BSB ? $"Result: FOUND - [{result}]" : "Result: NOT FOUND");
}

public class ClientAccountModel
{
	[BsonConstructor]
	public ClientAccountModel()
	{
	}


	[BsonId]
	[BsonElement("internalAccountNumber")]
	[BsonRepresentation(BsonType.Int64)]
	[BsonRequired]
	public long InternalAccountNumber { get; set; }


	[BsonElement("bank")]
	[BsonIgnoreIfNull]
	public BankModel Bank { get; set; }
}


public class BankModel
{
	[BsonConstructor]
	public BankModel()
	{
	}


	[BsonElement("bsb")]
	[BsonRepresentation(BsonType.String)]
	[BsonRequired]
	public string Bsb { get; set; }


	[BsonElement("number")]
	[BsonRepresentation(BsonType.String)]
	[BsonRequired]
	public string Number { get; set; }
}