mp3 album cover bug

I've just added mp3 albums to CDJ, before this all I had were the hundreds in my Sony changers. When I add a cover to an mp3 album, it works like any other disc. But when I exit CDJ and then re-open it, the links to all the mp3 album covers are gone, only the covers for the discs in the Sony players remain. I know the library had been updated because it saved the rest of the mp3 info, just no covers. Does anyone have any ideas on this, has it always been like this or is this a recent bug?