Hello,
You put the start point in the Plane input of the Polygon component, which requires a plane. Grasshopper therefore makes a conversion, and uses XY plane with the origin at the start point, which is why the triangle is flat.
You need to use Perp Frame. This component requires a parameter, so you can either use Curve Closest Point to calculate that, or reparameterize your curve and use 0 as the start parameter.
Then a Sweep1 works fo define your arch surface.
Follow me.gh (15.3 KB)