Just ran into a slight UI issue when using vertical tabs. You should be able to see what I mean by the screenshots, but basically the tabs content wrapper doesn’t fill the height to give a background to the tabs.
This has been reported a couple of times lately. I will mark this topic so that the developer will know to take a look at it if he has time. You might also want to create a new support ticket, that will likely get to him faster http://support.advancedcustomfields.com/new-ticket/