Problems with some visual residue when doing transitional flip between UIViews (video inside)

So I'm transitioning between two views here by replacing one of the views with the other, but I'm having some problems with some visual residue.

Both views have rounded corners set, and when I do the flip animation there's this rectangular shadow that appears on top of开发者_Go百科 both views. There must be some way of getting rid of this?

Here's my transition code. Transition view is a background view with a clear colored background. Tried adding the second button on top of the first and transitioning that way as well, but that also gave me the same problem.

[UIView transitionWithView:transitionView
                  duration:4.7
                   options:UIViewAnimationOptionTransitionFlipFromRight
                animations:^(void){
                    [clockButton removeFromSuperview];
                    [transitionView addSubview:motorButton];
                }
                completion:^(BOOL finished){
                }];

Added a YouTube video that shows the problem. Slowed down the transition from the first to the second button so that it's easier to see what is going on.

http://www.youtube.com/watch?v=TemHSL0dfJI