I did some more research and Microsoft has a lot of redundancy in their data. They write the same fields to multiple places (EXIF, IPTC, XMP) which is odd. They also have a bunch of data in their own namespace that PM does not display or edit. But while I can find out where they store things like "Authors" and "Tags", I can't see where they store "Subject". It isn't in the JPEG file anywhere that I can find.
Anyhow, I don't think PM will be of use to you for setting this mystery "Subject" field.
-Kirk