2013~2016 Car won't power off completely if iPhone is plugged unto charger?

My car was with the dealer as they could not figure out why I'd park but when I'd turn the car off and open the door I'd get a Set Power to Off message on the console. They claim that by having my iPhone plugged into a charge in the aux power port under the radio in the center dash, it sends voltage back and confuses the system. Supposedly, there is a U300 diagnostic code related to it. There are no TSBs, they said they found some entry in a Mazda database about this.

I can't find any information on the internet relating to this problem or error code. Can anyone verify this?

Hmm, well it is so that the US version stays powered up after the ignition is off so I guess it could backfeed. Curiously, the European models switch the power supply off. I take it that it works fine with the phone unplugged?

The power to the aux port does turn off, but the problem still occurs. Worse, you can't lock the vehicle when it is in this state. Took a while to figure out but you need to unplug phone, restart and then turn off again.

The dealer says that they use they have something that plugs into that port for diagnostics of some sort and that's why the iPhone can cause these problems. Sounds ridiculous to me. Although everything about bluetooth connections to iPhones has been a disappointment with this vehicle. Siri has never worked properly.

They did give me a 2018 CX-5 as a loner for 2 weeks while they tried to figure this out - so I can say they finally got this stuff figured out. New models finally support Airplay and Siri.

But, I remain dubious as this problem only surfaced in the last few months and I've had it (and my iPhone) for several years. And, I can't get any detail on this U300 code.

I will try turning off my car with the phone plugged in,
I usually always unplug it before, but i really think it'll shut off properly...