I defined a symbol and applied 9-slice guides to it. When I transform the symbol, it transforms fine (the 9-slice works the way I expect) until I flip the symbol horizontally or vertically. At that point, the symbol scales as if it had no 9-slice guides at all. am I doing something wrong?
Wow your right! I would suggest just making a duplicate of your movie clip and flip it inside the library. So you have 2.