This is going to be short and sweet. I had a detailed post that, by the time I posted it, produced an error (I think something timed out).
I am using the MQTT Client from LabVIEW Open Source Project to share data between LabVIEW and .NET. I tried to use the pub sub example. It works when publishing and subscribing within LabVIEW, but would not decode payloads that were published outside of LabVIEW. I discovered that Publish.vi adds bytes to the payload and Decode Application Message (Type).vi removes those bytes from the payload. I did not know how to deal with the extra bytes within .NET, so I replaced Publish.vi with Publish (Raw Payload).vi and Decode Application Message (Type).vi with Decode Application Message (Raw).vi.
This post is for future users (including me) who need MQTT payloads to work between multiple programming languages.