App presents strange scroll on iPhone 6

It takes a bit more information to give an accurate answer.

More "old" apps, which use XIB, tend to work because the XIB for iPhone 5 is automatically staggered on iPhones 6/6 +, as the aspect ratio (16: 9) is the same. >

Applications that use the old type of splash screen (Default * .png images) also scale automatically because they are presumed to be made for iPhone 5 or smaller. When you swap the static splash screen for an XIB or Storyboard screen, it is signaling that the app is compatible with iPhone 6/6 + and then the different screen size issues appear.

If you are using those newer screen-drawing features (Storyboard, AutoLayout), you have to rework the screen project so it adapts to different sizes by adding constraints. If the entire screen is drawn with absolute positions and sizes, exactly what appears in your picture happens: the interface does not occupy the entire screen. You have to start with relative sizes based on constraints.

At least this is my experience, comment if your problem does not fit ...