This has happened to me once. I have LTE. Parked in a garage with not great signal during the day, my battery was a little low, I'm guessing that's why the car went into sleep.
I tried rebooting a few times on my own and with a tech on the line. Each time I rebooted, I noticed that the bars stayed at the same level in a way that made me think that the system wasn't actually fully rebooting.
I did some quick googling and found the information about the USB port thing. At the time, I only had a Apple lightning cable plugged in, the cable wasn't plugged into any phone.
However, when I took out the cable and did a reboot, that time, the bars went to 0, and then came back with LTE.
I'm guessing that somehow these ports are finicky about not actually shutting down if it can't release the device. So instead of really completely rebooting, I'm guessing we may be just hitting a timeout and the car just loading the UI services again.