MS Word Numpages field does not convert to PDF correctly

I am using Win 7,  Office 2010, and Acrobat DC. I have many word 2010 documents with page numbers formatted as "Page [page] of [numpages]" where numpages is the field showing the total number of pages in the document. I always convert these files to PDF before sending them out to customers, usually using the "Save as Adobe PDF" feature on the Word File menu.  Suddenly this morning the numpages field does not convert correctly. The footers of a 6-page document show "Page 1 of 1", "Page 2 of 2", "Page 3 of 3", etc.  This has happened on multiple tries on several similar files. 

Oddly, it works fine if I use File>Save As  and then specify .pdf for the file type. Is this a bug?