What is a 90 degree shot up or down?
Is that when the barrel axis is vertical, or when the scope axis is vertical? They are not the same.
There is no position where the pellet appears to stay on the centre of the crosshairs; that would require the crosshairs to be in the barrel. So the pellet will always cross the line of sight. Tilting the gun up from horizontal and nearer the vertical just brings closer that point.
Always assuming of course that the scope has been set up properly in the first place.