I've investigated some more, and yes, we have changed something internally so that file attachments are now included in the data for items referenced in app ref fields. We didn't expect adding more data to break any client, but the .NET client is unfortunately not very flexible.
The current .NET client is user contributed and as such not officially supported or maintained by us. We are working on a new .NET client that will be released shortly. For that reason we are not going to provide a fix for the current client, but you are much welcome to fix it yourself and submit a pull request to get the fix merged into the client.