Push the down arrow without holding it. The entire bar disappears and does not return. The only way my iPad Pro will bring it back is by going into settings. It's far too easy to hit it by accident or not long enough. And if you forget, you will not see it again without resetting it.
Now if on others iPads it automagically reappears, then my iPad pro has bugs. But my two neighbors who also have the iPad Pro say it works the same for them. They hit that down arrow button on the first day, and didn't know it was available any longer as they never saw it again when I asked them about it yesterday. In fact they had forgotten all about it until I asked them, and now have it back.
My point, as long as it is turned on in settings and remains on, when you lower the window or go out of window and then come back, so should that bar.
I'd bet the majority of people out there have forgotten that bar is even available... NOt saying it doesn't work, just saying the acces to that bar is not what it SHOULD be. If I have it ON in settings, why does it remain hidden until I turn it off and back on again? I can see the use to turn it off in settings, but why off and on to get it back?