This has been an issue in filemaker go, even in 12, as far as I know there is not a fix for the issue. I would suggest using a tab or slide control and I would also recommend creating a separate layout for your computer and ipad, because not all items work the same and the screen is much smaller on the iPad.
Thank you for your post.
Our Development and Testing departments are aware of this issue. When a portal has a vertical scrollbar enabled, it will appear above other objects that are placed on top of it.
I have attached your post to the original report. When more information becomes available, I will post again.