Offline mode isn't working

[Sam1980]

Plan

Premium

Country

UK

Device

Moto G5 Plus

Operating System

Android (8.1.0) 

My Question or Issue

My Spotify app won't play downloaded albums when placed in offline mode. It will play downloaded playlists - just not albums. I get a 'Something went wrong - have another go?' notification. It will play albums when in online mode. It will also play downloaded albums when the app is in online mode and the phone is in flight mode - so it doesn't appear to be an issue with the actual downloads, only with the app accessing those downloads when in offline mode. It will play downloaded tracks from downloaded albums when in offline mode when accessed through the 'artists' tab. So far as I can see, it's just the 'album' tab of 'my library' which has the issue - which is frustratingly how I virtually always access music. 

Downloaded content is stored on an SD card which has plenty of capacity and is working fine in all other respects. 

I've logged out and in again, restarted the device, and cleared the app data cache. I'm not keen to do a clean reinstall if avoidable as I have a large library of downloads. 

Any ideas? 

Failing any ideas to resolve the problem - does the Android app play content which is downloaded from the local download location (e.g. SD card), even when a data connection is available? (i.e. can I just leave the app in online mode permanently, without it eating data by streaming content which is also already downloaded?) 

Thanks.