Issue with the transformation of a parametric shape

[B-Vs-V]

I start with a rectangle. I change its shape with the shape editor tool. I duplicate it and apply a blending. When I want to modify the shape, I get an artefact at one of the extremities (lower left). A bug?

Issue:

[VectorStyler]

@Pat It could be a smooth / cusp node flipping its state.
Send me the example, and I will try to figure it out.

[B-Vs-V]

@VectorStyler The file is sent.

[B-Vs-V]

@VectorStyler as additional information, this issue appears only for certain positions of the curves: