Why does rukia not disappear in the anime version after aizen's defeat?

I think it's because of the filler arc. Even though it's filler, it's been written as if it follows on from the events after Aizen's defeat. Because Ichigo still has his powers (although, they are getting weaker, I think), he can still see her - so she hasn't faded away. It's possible that maybe it will happen because Ichigo will eventually lose his powers (he'd have to so they can pick up again from the manga at the end of this filler arc).