I've read the manga recently, and it's a whole different world. Most plotholes that existed in the Anime are simply non-existant thereIv'e always assumed the Mystic transformation was indefinite and that his ability to go into SSJ was just a plothole in GT? I hope they give a valid indication why his power level turned turd-worthy in GT.
Man, the manga is SO much better Oh, and oh gawd, GT. As soon as we all forget about that thing, our lifes will be SO much better.