Sometime champions act differently under specific situations. The funny thing is most of the time they do follow some pattern that not always make sense but they change that behavior when there are some conditions involve. 

Things that may affect that may be when they are not fully developed, not ascended, not booked, fighting against different affinity, whether or not there are a buffs or debuff presented in fights at the moment on your or enemy team, position in turn order and things like that. 

Most of the time default auto AI for Spirithost starts with her A2, but there may be some specifics in your game that change that. And she is currently 3/30, probably no books yet, so... it can be one or more varieties involved. 

I think it's as simple as: if there are already any debuffs on your champions when she gets her first turn, because an opponent already got their turn, then she will remove those debuffs instead of putting her attack boost. Which to be fair is often the better move anyway. If there are no debuffs on her team yet, I normally see her casting the increase attack. 

So if you really need her to cast the increase attack, make her faster so she goes first...