What makes one Loft a Brep and another a Trimmed Surface?

I’ve split a Rail Revolve surface using lofted surfaces and am trying to subdivide the resulting split surfaces.

It works on most of the split surface sections but not on the last one.

These curves produce an Open Brep…

And these curves produce a Trimmed Surface…

Does anyone know why or if I can fix the elliptical curve and make it a closed curve?

Loft.gh (119.0 KB)

Different parametrization.

The top cut curve is much simpler than the other 3. The first one is degree=2, the others degree=3.
Even the untrimmed surface you are getting have kinks.

Simply rebuild all curves with enough control points and same degree. Loft will be smooth and much cleaner.

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Ah… Rebuild!
Thank You