User blog comment:Trainer Micah/Pokemon UltraSun/UltraMoon prediction's 4 Anime/@comment-30415965-20180122132904/@comment-26573805-20180202015627

I'll translate the summary for you: そこで、サトシがバトルロイヤルで最強のロイヤルマスクにニャビーで挑むことになったのだ!!
 * バトルロイヤルの会場に行ったサトシたち.
 * Satoshi and co. go to the place of the Battle Royal. There, Satoshi is faced to battle the mightiest Royal Mask with his Nyabby.

Seems as though it's actually evolving in the episode and staying Nyabby until then.