I did not have this bug in my Xbox game and I figured out why: I still had the scroll, forgot I had it when Dawnguard was released for the Xbox long ago. So I didn't have to buy it back. The option to sell it is only once so don't use that option until after Dawnguard. I can not drop it, sell it or put it in a bookshelf. The other 2 can be disposed of, but not the Dragon scroll. My only new bug in the DLC so far, not game-breaking, but thought I should warn people who can still be saved this annoyance.