Your problem was the unplugged alternator. By unplugging it, the alternator lost it's reference voltage and went to maximum which was 18.5 volts or more depending on the regulator version built in the alternator. You are lucky if all you lost was the isolator. The same thing would have happened if you had a combiner installed. The excess voltage and associated current caused by the disconnected alternator sense line had to go somewhere. Usually it is the battery(s) that most of absorb it until they boil dry. Then the voltage goes even higher.

I had that happen on an airplane that I was flying one time. It boiled the battery dry and then took out the radios and several position lights. I ended up shutting down the alternator in flight and flying home a couple hundred miles in the dark without anything electric working. Aircraft engines do not use any alternator / battery electric to run. They have dual magneto ignition systems.