I would like to report a serious issue with Spotify for iPhone. I don't know if this applies to other versions as well.
When playing music through headphones or AirPlay, it should automatically stop when either unplugging the headphones or losing connection with AirPlay. This works for Apple Music, but not for Spotify (at least not the current version, I think it worked before). When the volume of the phone is set very low, you do not notice Spotify is still playing in your pocket. If this happens outside of your home, e.g. no WiFi, Spotify will continue to play all day and consume your 4G data bundle. This typically occurs when unplugging the headphones after traveling somewhere, or when leaving your home AirPlay zone when leaving for work.
I have now already lost 3Gb+ twice, thus depleting my complete bundle twice, and bringing high costs of extra data for the rest of the month. please repair the standard functionality of automatically stopping playback.