Checking my app\'s behavior on M Preview device, it seems that its foreground (with active notification) service playing music is not affected by Doze mode.
In this post's comments on Sep 17 Dianne Hackborn writes:
Apps that have been running foreground services (with the associated notification) are not restricted by doze.
So yes, your findings seem to be the intended behavior.
Attention, Foreground service can prevent your app never into app standby, but not doze.
Here use adb cmd to test
$ adb shell dumpsys battery unplug
$ adb shell am set-inactive <packageName> true