Hello,
When I choose a DAC from the dropdown menu in the System menu it is correctly set or changed but when I choose None the /boot/config.txt file is not updated and the entry dtoverlay=xxxx stil remains.
No big deal as it does not stop the system from booting and workin, just an error message on the console.
One thing more annoying: it removes any line that was manually added, for example the max_usb_current=1 used on Pi2 to boost the USB ports.