This seems to be about right. I tried various things tonight before driving home.The radio and nav just appear to be two different apps within the infotainment system and you can run none, one or both at the same time. There is no off switch for the radio. Nav will overlay a popup at the bottom of the screen if you are running the radio and you can choose to view/dismiss the info. if you dismiss the radio screen stays up, if you choose to view it switches to nav but the radio audio stays on.
The "display: off" in the settings is basically just turning off the screen not the system so its like turning off a monitor and leaving the computer running so the mini-nav still stays in the DIC. (The same can be accomplished by pressing and holding the volume button.) If there is anything requiring input then the screen switches itself back on again.
I was able to reproduce this by leaving the nav running, then switching to audio on the main screen, then turning the screen off. When there was a turn prompt in the nav the screen came back on because the popup came up asking to view or dismiss and required my input. On the other hand, if I made sure the nav was the active app and turned the screen off then it stayed off as there was no popup. I also set the audio to aux to make sure no radio sounds. You can then switch nav sounds on and off in the DIC display while leaving the main display off.