Thanks for the info.
Groupings were, indeed, the problem. All the lost lines were in groups.
All drawings were 2D that, after a couple of more steps, are inputs to a laser cutter. I liked thing in groups so I could easily move them around to optimize time and material on the laser. The no groups requirement is inconvenient but not a show stopper.
Another issue I have not clearly defined is loss of dimensional stability, which is important to a laser cutter feed. Some .svg files hold their size perfectly while others become larger. I have not had enough time to determine what sequence of events drives a particular outcome. Solving this has to be done at the shop and it will be days before I can get there to sort the dimensional problem out one way or the other.