The bug is that amazon changed the way they process non personal documents a while ago. Now they try to download cover thumbnails for them from the cloud. That is fine. The bug is, when that download fails, either because of a not working net connection or because the file has an non-extant ASIN, they display a blank thumbnail instead of falling back to the thumbnail from the file. I have notified them of this bug over a year ago, they dont care.
I have wasted weeks of my life reverse engineering amazon's crappy ebook software, and even crappier closed ebook formats that they themselves cannot render correctly. At this point, I am fed up with them. You are welcome to beat your head against the brick wall of their indifference, I wish you the best of luck.