Transaction not being applied to operations

Hi,

I am using MongoDB Driver 2.13.1 and a cloud atlas cluster 4.4.10 M0 sandbox to develop stuff.
In a test i am doing two operation in one bulk operation surrounded by a transaction. Documentation states that if you abort the transaction none of the data will ever become visible. In my case a (deliberate) duplicate key exception still commits the first record to the database (visible to the outside world even before commiting or aborting the transaction). Alaso this first change doesn’t get rolled back.

My guess is the session isn’t applied to the operation. What am i missing?

 using (var session = await MongoClient.StartSessionAsync())
{
	session.StartTransaction();
	try{
		var bulkResult = await Collection.BulkWriteAsync(bulkOps);
	}
	catch (Exception e)
	{
		await session.AbortTransactionAsync();
		throw;
	}

	await session.CommitTransactionAsync();
}

the mongoclient is injected through DI

servicesBuilder.AddSingleton<IMongoClient, MongoClient>(s =>
{
	var uri = s.GetRequiredService<IConfiguration>()[Statics.MongoDB_ConfigPath];
	return new MongoClient(uri);
});

MongoDatabase;

MongoClient.GetDatabase(_databaseName);

Collection;

MongoDatabase.GetCollection<MyType>(_collectionName);