I literally just joined this forum to post the same exact issue. I'm sure like me, you've discovered that although the entire document takes on the characteristic you wanted to apply to only a selected piece of text, that UNDOING (whether by holding down CTRL + Z or clicking the undo icon) removes the formatting from the rest of the document, while applying it only to the text you originally wanted to affect. I find this to be unacceptable and unnerving, but I've literally come to expect this will happen when I try to format a selected area and now I almost automatically plan to undo what I've just done. How horrible is that?

Did you receive any more info or any other replies? I was thinking of uninstalling and reinstalling Word. If I do, I'll post my results. Take care...