Interesting... I get the same issue... Almost always shows the song that was playing at the time of locking the screen... It does get refreshed when he app is opened up again... This is not so serious as my current issue wih the app stopping all songs after 1m 30 sec... It renders my having spotify pretty useless
No update has fixed this yet an I've actually noticed new bugs since the last update. The worst is occasionally a track will not play or will stop playing. The pause button will be displayed (indicating the track is playing) but the progress bar will not move and I won't hear anything.
The mobile app needs a LOT of love. I love the service but the software (mobile and PC both) need work. Especially for those of us that pay for it.