Before installing newest firmware & SD card my iPod & artwork worked perfect on my 2012. 
I noticed other day apprx. 30 songs out of 2400+ were not playing with the correct artwork. 
Today I decided to try a new cable and sure enough it started on it's own to index all songs again. 
I now have all music correct with the correct artwork. Thought u guys that are having issues  might try this!