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The fillet is failing at the bottom right corner. The discontinuity element is working just fine and produces 4 corners. However, after exploding the geometry, it turned out that the outermost curved edge is being divided by the x-axis. The fillet is failing when its radius is larger than the tiny edge below the x-axis.
I find this really strange, especially that the segments output from the explode doesn’t describe the whole closed curve. Does anyone have an idea what might be the issue here?
Here’s my definition: Split edge.gh (7.3 KB)

Every closed curve (circles too) have a start and end point that are congruent; also called the “seam point”.

An unspecified “generic” circle on rhino (and other CADs probably) will be aligned on XY plane with the seam point on the right (X+) of the circle. (and the circle is oriented counterclockwise)

Your curve contains a big arc that is not a single object but 2 arcs with same center and radius.
Probably those 2 arcs were originally a single circle that later was trimmed, resulting in a split aligned with the X axis.

If that curve was made on grasshopper you can edit your construction to avoid this…

The seam point of the original circle from which the large arc was trimmed was indeed the cause of the problem. It worked after modifying the seam point (using the “Seam” gh element) and aligning it with my geometry.