I've seen this issue mentioned in a few threads, but I'd like to raise another flag for it since the problem has still not been fixed.
I'm running Spotify 18.104.22.1685.g943b26a8 on Windows 10 and I mostly use the Spotify catalog, but I have some local files too. I've meticulously embedded album art (500x500 image/jpeg) in all my local files using Mp3tag and the Spotify desktop app refuses to display some of them, seemingly at random. When I play an affected track, either the album art display is empty or the album art of the previously played track is shown.
When I sync those files to my Android phone (via playlists), the album art works perfectly on the Android app! There is clearly an issue with the Windows app.
I'm happy to provide more info or assist devs with debugging if it means this will get fixed. 🙂