I guess you could call it a bug, but it due to fundamental differences between the two platforms.
iPhone uses a single page horizontal format.
iPad uses a two page more vertical format.
When you take a iPhone layout and open it on the iPad (or Mac), you see two pages instead of one, and the horizontal page is smushed into the more vertical space.
I recommend developing on the platform intended for use. There are too many differences between the iPhone and iPad/Mac environments for layouts to move without lots of manual reformatting. All the work you did on the iPhone is still there, you need to do some reformatting to fit the iPad / Mac format, mostly splitting the controls from one wide iPhone page to two vertical iPad/Mac pages.
Dan is cogitating on the future of the iPad/Mac vs iPhone dichotomy, but we need to wait until his thinking is ready to discuss.