When i do a contour without a starting Point, the boundary surface computes normally. But when i put a starting Point in the Contour parameter, the boundary surface does not compute and projects itself to the XY Plane. Check picture.
Well, yeah. You are giving Boundary Surface many different curves at different heights and asking it to create a single planar curve. It has to choose some plane for that, so it chooses XY.
You need to look more closesly at your data structure. You provided Contour with 3 different points. It’s going to create a tree with contours all in disarray, so that Boundary will not know which one to put with which.