Proportion: lofting errors from parameter direction
Here we have three lines, with one segment attached to each line. The the third segment, the one on the right, has a direction opposite the other two.

The blue rectangle is a correct loft, in which the parameter-0 points on the two segments can be connected by a line that does not cross the parameter-1 points.

The two green triangles are produced by the lofting of the second and third segments. Since the segments' direction do not match, you get a bow-tie pattern instead of a rectangle.

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