surface subdivison collects the wrong data after cutting something off a surface - what to do?

I want to make a loft, an offset and SurfaceSubDivisions for both of them for an easy folded structure.

The original loft starts at the xy plane. The offset, which defines the depth of the folding has starting point below the xy plane.

Then I tried to cut off the parts of the offset that's below the xy plane:

with a surface split. it worked, but....

with a closed loft and and boolean operations. it worked, but...

....afterwards I did a SurfaceSubDivision to get the points for the folding structure and the surface sub division just took and divided the original surface.

it didn't care that i actually cut of parts of the surface and defined a new different surface, that I wanted to subdivide. Seems like a mean bug to me.

I tried to deform the offsetsurface, but it only moves points and is very heavy to calculate for the CPU.

The surface can hardly be described differently than by a loft.

I also can't make a second smaller loft, after having sections between the 1st loft and a series of xy planes, because at some point the surfacve of the loft moves almost parallel to the xy plane.

Maybe some has ideas. to Work around that bug.