I guess I have successfully solved that. Somehow predicted this problem's appearance and added a function of ID3 tags version conversion. Just convert all of your mp3 files' tags to version 2.4. worked for me, now bitrate gets downgraded and artwork is safe.
p.s.: some of the artwork (especially files that originally were higher bitrate AAC converted to 128 kbit/s) won't show the artwork in the artist or album list at first. just play any of such songs and artwork will show up.