Try to press and hold the power button for few seconds to turn the unit off. per manual~
I'm having another problem with my '18 Limited's HK sound system / head unit:
There is no way to turn it off.
If I'm listening to music from any source - radio, bluetooth device, Apple CarPlay - pushing the power button to turn off the system does not turn it off, it MUTES the system. You see the "mute" icon in the upper right of the screen.
This is a software glitch which I hope can be resolved with an update.
If you own an '18 Limited or Touring, does your power button turn off your sound or just mute it?