Spotify on iPhone went nuts...

[peebees]

Hi, last week I connected my iPhone (iPhone 7, iOS 10.2, Spotify 6.8.03786)  by USB cable to my Alpine car radio, as I usually do when I go for a drive. Strangely I was logged out of my Premium acount in Spotify and had to re-login. I can't remember that ever happening before, but I re-logged in and drove to my destination, enjoying my playlists. Later that day I received a warning that my mobile data bundle was spent. Spotify had obviously lost connection to all of the offline music stored on my phone, and re-downloaded 2.400 songs over the air (4G). Not so nice: 3 GB gone and a month of no mobile data to come.

Does anyone know: what happened here? How can I prevent this in the future?