I have done Siegfield's quest several times now, first two times everything went well and the reward was what it should be (a pretty shiny book of shockwave)

If I try doing the quest again, the book I pickup seems to randomly mutate into some other book (icestorm, amour, snowstorm)
The mutation seems to occur on identifying the book in my inventory before it ever touches the ground. Mutation continues if the book if left alone for sometime on the guild grounds. Sometimes the books turn back into shockwave, at the moment I have 2 books of shockwave and 3 of snowstorm.
I am aware that mutation of spellbooks in apartments is common knowledge, but this is the first time I encounter mutations of quest spellbooks.