I've been using AA via USB on another car since Android 9, maybe even 8. I've never had to launch the app or do anything in the car; upon plugging in, everything started automatically.
The only time I had an issue it turns out I had a little fluff of pocket lint stranded in the USB port. Picking it clean with a sewing needle solved that.
I've only used the wireless AA connection on the ID.4. Same startup though: I power up the car and it starts automatically.
I can't imagine your One Plus is any different. Sounds like a cable or port problem, assuming you've successfully completed initial setup, which only needs to be done the one time.
The only time I had an issue it turns out I had a little fluff of pocket lint stranded in the USB port. Picking it clean with a sewing needle solved that.
I've only used the wireless AA connection on the ID.4. Same startup though: I power up the car and it starts automatically.
I can't imagine your One Plus is any different. Sounds like a cable or port problem, assuming you've successfully completed initial setup, which only needs to be done the one time.