Hi Booleano, when you have a joined curve, sweep will try to do mitered corners.

That's done by extending surfaces and then intersecting them with each other which becomes a difficult calculation if your curves are close to being tangent to each other but are off by a little bit as is the case in your curves. When you have curves like that the extensions do not have a crisp intersection.

In your case here the problem is that these curves segments are not tangent to each other, they have a shallow approx 3 degree deviation in their tangents:



- Michael