just installed the beta, did a restart and it turns out, the error is still there.
One interesting aspect to note here:
I figured out, if you resize the taskbar (small, medium, large) the error gets worse the bigger the taskbar is.
Soooo i thought i'll try auto hide. Gues what, error is gone. (disable auto hide and the error is back again)
My best guess is, windows thinks if the taskbar is on top, that it's actually hidden. Because the position of the quick settings menu is significantly lower than it would be if the taskbar is on the bottom.
So it calculates, based on a hidden taskbar, but actually get's a smaller working area because it's on the top.
Maybe this helps the devs somehow.