Not sure if it's a bug, but if you create a button with a background image, and round the corners, the background image is not rounded.
I attached an example with border width 1, and corner radius 10, so you could see
this is the default behavior of iOS buttons, you can get rounded corners for background image by setting the
clipsToBounds property of the button to
true. In the current release, this property can be configured only programmatically, but we will add the possibility to configure it in the IDE.