I found this on the GOG forum, where folks were reporting bugs they had experienced with the game:
Auto-save is broken. For some reason it saves automatically at loading a custom saved game. So when I loaded an old save, I lost my autosave with a lot of progress.
I don't know if this pertains to your problem, but unfortunately it sounds like it could be a bug. I don't know if any new patches have been released for this game.
Maybe someone else will know more.