That's how it's suppose to work with auto-hide turned on and off.
Since the August 12th 2014 Windows Updates in Windows 8.1, the taskbar will always show on the Start, apps, and PC settings screens if you do not have the taskbar set to
auto-hide and you open the Start screen by pressing the Start button on the taskbar while on your desktop. If you multiple screens, then you can click on another screen to hide the taskbar.
If you do not have the taskbar set to
auto-hide or you open the Start screen by pressing the Windows :winkey: key, you will not see the taskbar on the Start, apps, and PC settings screens unless you open the taskbar.
How to Open the Taskbar in Windows 8.1
I'm a little confused here because, yes, when you click the start button, the taskbar does show up on the start screen, however as soon as I move the mouse away from the taskbar it auto hides... even though I have auto hide disabled
Anyway I think there are two different things going on here and thus causing some confusion...
1 - when you click the start button you are taken to the "Start" screen, there the taskbar is shown and than auto hides when you mouse off of it.
2 - Now, when you click the circular down arrow on the Start screen, you are than taken to the "Apps" screen, where you can scroll from left to right, and... you will get the taskbar if you mouse way down to the bottom of the screen, but moving away from the bottom here will return to the scroll bar.
3 - I'm also sure that if your Start screen is full of apps requiring a scroll bar, there will be one there, if not, no scroll bar necessary.
At least that's the way I see it.