Those gauges are usually electromagnetic type, two coils, one for field. This makes them self regulate with system voltage changes. Many of the at type simply float anywhere in the absence of any power applied, but different designs of course may act differently.

It is possible that the reading voltage is removed before the field voltage, which would cause this.

In many cars, the ACC voltage is not directly controlled by the ignition switch. There may be a slight delay in the computer turning off that voltage.

Actually I prefer cars the old way. When you turn the switch on the light goes on, when you turn it off the light goes off. i don't need the radio and windows to work after the key is off before a door is opened. One day I will buy a car and it will be an old one. The older the better, to a point. by about 1970 they got the suspension pretty right and they ran right. newer cars are detuned to keep O2 in the catalytic. If they ran at stoichometry the light would go on.

I had to get rid of a couple of cars because of wiring/electrical problems. It is ridiculous.

I also had one car in which sometimes the tach would run backwards. it was a Dodge, which I usually don't buy. It wasn't mine I just drove it. We used to just grab whatever was last in the driveway. Another thing they changed, we had bonds and were covered on any car period, now you have to pay extra for that. Another thing that is new and improved.

But I am pretty sure about those gauges, I am pretty sure if you leave that car sit for a few days especially on a sideways grade those needles would float into another position. But don't try it, the wheels might fall off. LOL