It is the nature of plywood that it will warp of it's own accord if not well supported/fixed
For some years I have been using ply on flat roofs where it is expected to survive at least 30 years.
We always regarded 416mm as the maximum safe span for 18mm ply.
So I am not surprised at the 9mm cupping.
The " hardwood" ply sold here in Ireland is visually very pleasing but now has a bad reputation in the roofing trade because it tends to delaminate .
The birch is usually very good quality ( especially the scandinavian ones ) but it still needs to be well supported/fixed