A family of Microsoft word processing software products for creating web, email, and print documents.
Yep, quite right Terence. It's a Word 201x; problem and nothing to do with Windows 10 (not in my case anyway).
Was having the same problem on my Windows 7 laptop running Word 2013.
Read your post, selected Devices and Printers, Manage Default Printers. Turned off "Change my Default Printer when I change Networks" by selecting "Always use the same printer..." instead and, hey presto, envelope addresses correctly aligned.
Doug, sorry, but MVP status withdrawn and not just for being unhelpful but for being quite unnecessarily rude.