But instead of the Back button I inadvertently hit the Home button and was taken to the phone's Home screen.
After starting Automagic again I was in the list of flows and, after navigating to the last edited flow and opening the script action, I found that all changes to the script were lost

If I had used the "Recent apps" screen to switch back that would not have happened, I checked. But the Automagic icon sits right on my home screen, so that was faster (I thought).
Shouldn't Automagic show the last activity when started from the app icon if it is already running?