Sorry Bier, you may have run into a bug. I get the black area in all ordinary stairs (created with Stairmaker's tools) and I also get one when I tried to create a similar stair to your one, but in this case the black area is misplaced (as I see it) and not anywhere near the winder range. And if I try to move it, it just blinks on me, and the stair gets weird if I edit. So I guess something is wrong.
On the other hand, the stair I created this way looks fairly right to me, and I even think it would pass your code, which we don't have over here.
And come to think of it, I've never really been lucky with editing custom-shape stairs before. When I've got them all right, I've most often had to edit the original shape a couple of times to hit the target.
Anyway I think you should report your finding as a bug through your reseller. Something stinks around here!
If you can't get the black area back ay other way, i guess re-installing is the only way out.
AC4.1-AC26SWE; MacOS13.5.1; MP5,1+MBP16,1