I don't believe that Apple has added full Office 2013 Word support to Pages v5.6.1. If there are any Office 2016 (Windows or Mac) nuances that Apple have not addressed yet, then those could interfere with formatting when opening one of those documents in Pages v5.6.1 (or later). Unfortunately, with Pages v5, there just are no guarantees when opening any Word document.
I have a Word (.docx) document, a one-pager, sitting on my Desktop that Pages v5.5.3 exported, and though Pages v5.6 was able to open it, Pages v5.6.1 will not touch it, proclaiming that “foo.docx cannot be opened for some reason.” Yet, Pages '09 v4.3, Preview, Bean, LibreOffice, and TextEdit all open it without issue.
I am about to bite the bullet and purchase Office 2016 for Mac.