The dragon is randomly generated like 30 seconds after you level up, as long as there's no dragon inside (you got the dragon from the shield on the previous level), i guess it differs by something else too, since it was ice dragon for me
EDIT: Ok the save i gave you the shield is about to change his dragon. If you open the hero screen right before the "You feel a sharp blow...." message, it'll still be the bugged shield. (you have about 2 seconds after you load lol >< sorry), I can provide an earlier save if you want.