I already suspected that.
I do not unterstand that they keep these bugs
in main features within their releases for years.
On a second look, with all the other bugs they do not seem to care.
Possible Sollution for anyone of you:
It works if you have Movieclip A with 9slice-scaling,
scale it (without rotation)
and wrap it into another Movieclip B, which you can rotate.
For my needs, the Movieclip which is scaled is the one that needs to be rotated, too.
So, sadly, thats no sollution for me.
Any other workaround known?