In every other car I've owned, if I press the volume knob to turn off the radio, the radio stays off. The next time I start the car, I have to press the volume knob again to turn the radio back on. In the Subaru, the radio turns on and starts playing when I start the car, even if I specifically pressed the volume knob to turn it off before turning off the car last time.

I encourage you to go try this in your (presumably non-Subaru) car. Start the car. Turn on the radio. Turn off the radio. Shut off the car. Start the car again. Is the radio playing audio or not?

As for the freshness of a renter's UX observations, personally, I'd find it less reliable due to different objective vs. a future owner. I would dismiss a lot of seeming inefficiencies of design just due to my unfamiliarity with vendor 'concept' and would kludge it for the short duration of the rental.

On the other hand, if shopping for a new car, test-driving one, my mindset would be a lot more critical.

I remember, at one time having to pull over the highway just to figure out, where was the headlights switch on the fine Dodge Charger. Used to Japanese cars, I would not expect it to be off the steering wheel stalks, but as a knob near the door. Turns out, it's another 'concept'.

Another example is the hand-brake pedal (!). Makes sense in a way. That one entered my memory bank via a nice Toyota Prius, at 4 in the morning, starting my the way to the airport from the darkness of parking lot. Previous share driver left it engaged, manifested itself with a dash warning and very squeeeeky and slow drive.... Luckily, there was a user manual in the car.

I have a 2016 Hyundai Tucson with a similar, but slightly different feature.

Turning the volume all the way down before switching off the engine does not effect the startup volume. When the engine starts, the radio comes back on at a preset, but reasonable, volume.

However, if I turn the entire screen off, by pushing the volume knob in until it clicks, before stopping the engine, then on start-up the radio will be off, but so will the entire screen, which also controls navigation, bluetooth connectivity and the reversing camera.

My first action after starting my car is 90% of the time to turn the volume down on the radio.

However, I must say that's really the only niggle I have with the Tucson. Everything else I'm pretty pleased with. While it does have a touch screen, I hardly use it, as everything can be controlled with tactile knobs and buttons. The touchscreen is used most when stationary and inputting a new destination address for navigation.

My Subaru has the same behavior, minus the backup camera remaining off. In reverse, the screen turns on and displays the image from the backup camera. It turns off again once you switch into drive. It's certainly a solution to the problem I described, but it's obviously not ideal for all the reasons you've enumerated. Turning off Bluetooth, navigation, and (in my case) Android Auto is certainly a drastic way of dealing with this radio behavior.