Word addin, set filename

Question

Hello,

If one starts a blank file on Word you can usually see in the top bar a name such as "Document1", "Document2" and so on.

Yet, if you attempt to open a file using the Word JS API like this:

   Word.run((context) => {  
     context.application.createDocument(documentB64).open()  
     return context.sync()  
   })  

The top bar comes out like this:

No filename is set.

To set a filename/handle(?), I tried using the code given here https://stackoverflow.com/questions/50275496/office-js-add-customproperty-to-new-document but that didn't help.

My addin is usually used in conjunction with another (VSTO) add-on and that add-on can't work properly with the documents opened by my addin and I believe the lack of a filename (/handle?) explains it to some extent.

Is there something I can do about this?

Thank you