Hi Pekka,
Sorry for the delay in getting back to you. I help out on SCN in my spare time.
Is this still open?
If so: Any chance you can share the XDP and a sample data set? You should find my email address if you click on my name on this post.
There are some limitations in accessing field instance across multiple master page instances.
Looking at your form, if you would have asked me how to structure the form I would have said that the address/invoice number should be on the body page and not the master page.
Best,
Juergen