And then I select, what is already selected? In my screenshot, "Center Aligned (experimental...) is what is currently active, and as you can see, the start button remains on the left.
But never mind, I figured out now, what the issue is: I have 3 monitors attached, but because one of them is off most of the time, I did not take that note into account, that only two monitors are supported.
So I rephrase my question: could this be implemented to work with more than two screens attached?