It appears that on iOS some people are finding album art is not being displayed for offline tracks. This is predominately found with locally synced files but has affected some users with Spotify streamed content set as offline.
Sync a playlist as offline and then play the track. No artwork is displayed
If you are having this issue, could you please let us know:
Model of device and iOS version
Whether the affected playlist contains only local files, a mixture or is all streaming only content
Any particular artists/tracks being affected
Format of the local file including encoder (if known)