This occurs in the latest update.

Issue: when rebooting the phone spotify data is lost. 

After rebooting my phone.

#1 log into Spotify again

#2 must move location of storage back to external sd card.

#3 must select music to be stored for offline use again

It is extremely annoying because it takes a long to to download again.

Analysis of problem

It appear on reboot Spotify service starts and doesn't find the SD card because it takes 20sec to 5 minutes to prepare the external sd card for use even though I use a super fast Samsung class 10 sd in my phone.

The large size of my sd and the fact Spotify makes thousands of small files to save the downloaded content makes the initialization of the SD much slower to worsen the issue 

It appears Spotify fails to find the SD while in preparation mode and defaults to internal storage and is stuck there.

As login is required to change storage to external sd the old data is just left there.

Old version of Spotify didn't have this issue if I didn't open Spotify until the external SD card was ready.