Hi, the other day I experienced this issue and since it's not something that is straightforward for the QA team to find I figured I'd share the bug and the repro steps here.

Summary

If the phone doesn't have connection or is in airplane mode and you change the SIM card you get automatically logged out of your account and cannot access until the phone has connection again (once it has connection there's no need to login, it does so automatically).

The problem is that the downloaded music is soft deleted, that means that the reference is lost but the music is still downloaded in disc so if I want to have the same songs downloaded for offline playing, they would take twice the disc space (old ones and new ones).

Repro steps

1. Download a playlist for offline playback
2. Go to Android Settings > Applications > Spotify > Storage and check the total sum
3. Set phone to airplane mode
4. Change SIM card
5. Open application 
6. The user is logged out of the application
7. Turn off airplane mode and reopen app
8. User is logged into the application but all the downloaded music seems to be gone (the option to download a playlist that was previously downloaded is available again).
9. Go to Android Settings > Applications > Spotify > Storage and check that the total sum is the same as in step 2 but the music isn't accessible.

Spotify version: 7.9.0.746 (Premium account)
Device: Samsung Galaxy S7 (SM-G930F)