That's how RavenDB generates IDs. Its a system document. Don't worry about that.
What's probably happening is that you are saving this document to the database that NServiceBus is using, but you are looking at either the RavenDB System Database, or you are writing it to a separate one.
In Raven Studio, check the "databases" list in the upper-right corner.
In your code, you can set the database name either in the connection string, or as a parameter to the new DocumentStore
constructor, or as a parameter to the OpenSession
method.