No volume control unless in Spotify app - using Chromecast Audio

[Jaemsco]

With both airplay and bluetooth audio casting, the phone’s volume buttons change the music volume, whether the iphone is alseep in my pocket, at lock screen, or in app.

Just bought Chromecast and on Ios 11.0.03, and all apps current, only in app volume works.  Please fix this, as it ruins the control and the point of casting music.