I found that flattening the scan of a shoe last with squish (in order to be able to draw 2D patterns), works much better with a mesh made by Quadremesh of the scan, then with the original scan. I will test some more, and ask the people of Jevero (2D pattern design plugin for footwear) if they are looking at this. I know they are eager to have a reliable flattening method, and that squish was not good enough sofar. Maybe Quadremesh is an answer!
There is still this strange bug that makes the flattening unselectable. I work around this bug by using _SelLast directly after squishing and moving the flattened mesh a bit. That makes the flattening selectable again.