I am using Windows Phone 10 and Spotify Version 18.104.22.168.
When I try to save the playlist or album offline, it downloads it and few minutes later, Spotify forgets that it downloaded and I have to re-download it. I believe the app deletes somehow the addresses in the memory since the re-synchronizing takes few seconds (independently of the internet connection). After more time, all the data would be lost and would need to be downloaded again.
Please help me! I need the offline mode working when traveling by plane and having to internet connection.