Recommend you check the brushes inside the SG and replace them. When they get worn down, they can sometimes hang up in the guide. The SG will still start the engine, but it won't charge. Also get a vacuum and a scraper to clean out brush filings buildup.
On the original topic involving the voltage regulator trying to start the engine after tuning off the ignition switch, I had this happen to me years ago. While panicking and trying to figure out what the heck to do, I turned the ignition switch back to the on position which fired up the engine so that the SG started charging again and allowed the VR points to open backup so when I turned the switch back to the off position the engine stopped.
On the original topic involving the voltage regulator trying to start the engine after tuning off the ignition switch, I had this happen to me years ago. While panicking and trying to figure out what the heck to do, I turned the ignition switch back to the on position which fired up the engine so that the SG started charging again and allowed the VR points to open backup so when I turned the switch back to the off position the engine stopped.