Spotify downloading over mobile network. BUG!

[guitargeek89]

So this month my telstra bill is going to be an extra $20, because spotify decided to update my discover weekly playlist over mobile network. 

I was listening to the playlist and it began syncing to my phone on my home wifi, I left to go to the shops and it continued downloading, even tho I have it set to only download over wifi. 

Has anyone else had this bug? I can fix it by restricting background data in android, but that will stop it being able to stream music while not on wifi. 

Does anybody have a fix? or is this a one off? 

Thanks.