The problem with your model appears to be in the boundary blend definitions. If you flatten just one of the triangle, and place a point along a long edge, you will see the end of the sheet is no longer tied to the point at the square. Lack of tangency control can do this.
Using the same curves, I made single blend with control points. Couldn't get a full single feature, but over half defined properly. The extrude is squaring-up trim. A point along the straight edge allowed for a reliable flatten quilt.
