Good evening Michael,
I tried to emulate the same issue but all the test I did created the same open edge. Maybe you can give me some recommendations on how to reduce this issue from happing. I know to close the edge on the hard edge is very simple what becomes more complicated is the curved sections. I kept the lines and main objects that I swept before doing the second action which is what is causing the removal of the edge.

I was able to keep the edge when I extruded the line shape and then Boolean it. However once that is done the edge does not like to be fillet again causes all kind of artifacting.

Again always thanks for your assistance you make this program what it is, thanks