When you add an Mp3 file to the WMP library it reads the tags and builds a database record for that track and from that point on WMP reads the database and not the tags.
So if you use Mp3tag to edit the tags for a track that's already in the WMP library, you'll change the tags but the old data will still be in the WMP database.
What you can do if that happens is delete the track from the WMP library and then add it back in so that it builds a new database record.
Better yet, don't use WMP. 