I’m going to assume that this is a quirk when using the Drape tool to drape lines into a flate plane.
I have a flat large rectangle which is my site. I’ve drawn some car parking spaces and grouped the lines. When I drape these onto the flat rectangle plane, they do create the lines I require, but the main face of the large rectangular plane is deleted. The outside edges remain.
The parking lines group is being draped onto the edge of the large group, so maybe that could be causing it?
Just seems odd that it does this.