Interesting - as you say 'Menu -> Info' shows the tags are still present. If there isn't an apparent option, and it was doing it initially, but now isn't... I am tempted to say it could be a bug - possibly triggered by something (maybe something got corrupted in one of the files, or data-file the headunit uses.)
Unfortunately I, like many AU users, can't really assist too much - as, over here, we get a totally different OEM headunit (colour- and touch-screen) with different firmware.
Excepting maybe asking if there is newer headunit firmware available at next service.... All I could really suggest is trying to delete the mp3s from the headunit and seeing if it changes behavior at somepoint. The other, last resort-ish, would be to do a factory reset (if possible) - unfortunately I don't know how one would do that and/or what the outcome would be.