Use projected curves onto the flat to get the lines to follow the surface bending/unbending.
I think there isn't a way for the software to manage a slot that has no sides. A rip doesn't exist in a real flat pattern, so it doesn't have sides, but the software can't display it that way. I expect that software that appears to do this only does it by adding the location of the rip to the flat pattern as a curve, not a solid feature.