I purchased MM vols I and II during the previous build of U15. Only Vol II pages showed up. Now with the new build, I can see that MM Vol I is listed as "not purchased", but it's no longer in the store for me to buy, either.
Perhaps related - during the U14 beta, Lord Points-a-lot broke my account. I was Premium before using him; after getting my points from him the first time, my account was downgraded to Free. I could still access MotU so I didn't worry about it at the time, but now it might be relevant.