The game’s input system lets characters jump cancel into specials, such as d, df f, uf + P to do a jump cancel fireball with Ryu. It has an application in that it makes you unthrowable for a frame but it’s mainly in there as an input buffer so that specials still come out if the player over compensates on the QCF motion. Every character can do this.
She has no jump input buffer leniency on her fireballs whatsoever. This means that if you hit an up input at any point during your motion, she will jump. You can set the dummy to do d, df, f, uf + P with Ryu and Hadouken will come out. Switch it to Poison and she will do a jump forwards and an air air attack. Sometimes, the attack doesn’t even come out and is eaten up by the jump frames, yet in the input history you will clearly see it there. Exceptions seem to be DP, Rekkas, Chickenwing and her Super and Ultras.
Doesn’t seem like an intentional feature, as her other attacks are easily capable of it, and more of an oversight. Seems like jump being higher than fireball in her input hierarchy may be the cause. No real way of knowing, but it’s a strange and detrimental quirk of her design.