there seems to be a bug or an unhandled type of string line.
The VPI definition of the source line is copied over to all the new segments and starts at the new zero of those new string lines. Hence messing up the elevation definition of the new lines.
The new segments don't have elevations in the horizontal tab, only in the vertical tab. But the "distance along" always starts at 0. The distances should be shifted according to the segment start distance in the source string line.