The only way I can get it to work is to not use an ellipse.
Stairs seem to only like a constant radius for the curves.
So can you re-create the ellipse with 3 arcs instead?
It will create a node at each of the arc end points.
You will need all of these stair nodes to be 'winders' with equal goings.
You might have to turn some of the rules off for things like max going length and max 2R+G so the stair fits the length of the arcs.