Probably an odd behavior, but just dropping it here because of having the same Issue and probably someone in the future might need it.
The only thing I could observe was that when I dragged into the "storyboard canvas" an UIView and changed it to the UIButton-Subclass, the Ctrl-Drag behavior was as if it was an UIView.
However, if I dragged into the "storyboard canvas" an UIButton and then changed it to the UIButton-Subclass, the Ctrl-Drag behavior was like that of a UIButton.