There's always the possibility that The Ninth was not part of the Ascian's plan, and something else messed it up.
7.3
Recent interviews with YoshiP suggest that the remaining Ascians are not going to be involved in a Rejoining-like plot so there could be other reasons for The Ninth's shift towards lightning. I might be wrong, but it was suggested that when The Thirteenth flooded with darkness, it caused a ripple effect to the other shards, corrupting them. It just might be the case with The Ninth, and the Ascians didn't bother with it because their focus was on the The First.