It was stated in Chapter 20 around pages 16 when Black talked about the more his cell are damage and recover, the more it becomes his own, which is how he gets stronger. So it isn't headcanon, it's in the manga.Basako wrote: A new soul? And where was that stated? Ok, my explanation for the different Zenkai is headcanon, but yours is too.
It's not only the hair, it's also the aura, it's totally different. As there are differences in the hair and in the aura, I was considering that other differences were for the same reason of being a god in a saiyans body too, but I can't say it as a definitive. It's still the best theory I see, better than being a new soul. Neither the body nor the soul are new, in fact, it's much older. Nah, I don't see this.
But, as I said, the rose has been shown so little, we just don't know if it has the same flaws or not. I didn't assure it didn't, but I wouldn't consider a problem if it was the case. Not gonna raise my plothole finger for that, as other differences have been seen without any problem.
In any case, I've always said that it makes little sense to me that Black has the rose form. If it was for me, he would have had the exact same transformations, the blue instead the rose and all the same advantages and flaws. But, Toriyama spoke, that's divine word.
Future Zamasu said that when a deity surpasses Super Saiyan God they gain pink hair instead of blue. So Super Saiyan Rose is Super Saiyan God Super Saiyan except the hair color is different. So it is odd that Super Saiyan Rose doesn't have the same problems as Super Saiyan God Super Saiyan since it's the same form and neither Zamasu or Black state that it's more stable because Black is a deity.