The vertical transition is from the last riser before the rail starts to run horizontal and turns from a handrail to a guardrail. When the stair made a kickback it ramped up turned and immediately started up the next riser but there was only one riser so it transitioned vertical and then to horizontal.
This is the top most landing where the servants would have stayed in the attic space. In the American South during the post civil war area they lucky they got a handrail at all.
I do like European stair design better than American design. They tend to leave a full tread distance at a landing or a kickback which gives a much more graceful twist and turn. The Americans always jam them together which always results in a vertical rise at a landing.
