Need help? Check out Spotify Answers for solutions to a wide range of topics.
I have Iphone 6 and was connected to a wifi and dowloading many playlist.
I receive a texto from my compagny of cell and in 1hours or 2 hours I take 3 go on the 3g instead of the wifi... and yes wifi assistance is off.
I don't have a clue, but to be sure this won't happen again, you can disable Spotify from using any data of your 3G dataplan. On your iPhone, follow these steps:
Settings > Cellular > Scroll down until you see Spotify, make sure the Spotify toggle is turned off.
Hope this helps, keep on listening!