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.
Except Poison.
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.
I wondered why I sometimes jumped. I mean I know that the input could’ve been better but i’ve literally lost matches because of it.
Literally just happened. Rose opened me up because of it. I ate like 80% hp in mixups and chip.
I’ve had this happen to me a few times doing rekka’s or fireballs and thought it was just rust from not playing the game for ages but then I play Dan and have no problems doing all my old combos for him. Thanks for this.
Now I have an answer for my rage questions of “WHERE THE !@#$ WAS MY FIREBALL?!” and "WHY THE !@#$ DID IT JUMP? I WANTED FIREBALL!"
Was seriously pissing me off that.
I usually add the up forward to make sure I input everything I need for the special move. 
Didn’t feel like I was doing anything differently.
I’ve already adjusted to not doing the extra input now.
Oh thank god it’s not just me.
And it’s not even like how it usually is if I accidentally TK something, because Poison just jumps straight up instead of forward.
thank god its not just me, half the time i do a fireball i end up jumping i just thought i was terrible at it, but i can always do it with Ken but not Poison
LOL i guess we all just found out that we weren’t alone in this weird buffer issue 
That’s why I jump when I input a fireball…
That’s not her only weird input priority strangeness. I was practicing combos tonight and notice that even on a correct plink of s. Mk (with LK) you can get a cr. WP to go into far LK rather than standing MK. This was with inout display on and it showing a proper plink with two medium inputs and the light input on the same line as the medium.
That happens when plinking mediums with lights. What is happening is that you are mistiming the plinks, and cr. LP is chaining into st. LK. You have to do it later to get st. MK to come out.
It happens with everyone. It’s terrible when it does happen, too, since it’s pretty hard to recover from mentally. You’re thinking “hey, I didn’t want that” and the other guy is moving already.
sometime her super comes out when I go into rekkas. Its very odd…
I ALWAYS JUMP… I WAS LIKE DAMN I SUCK…LOL
Actually a huge issue. Massive oversight. I’m a little baffled that nobody managed to catch it during testing. I’m concerned that there are other QOL issues that are gonna plague a lot of the new cast members.
I wonder if it’s a bug that occurred in this specific build, because there really is no way that it would have gone through this much time in arcades and location tests without anyone noticing until the day the game launched on consoles, so I feel like it had to have at least happened after the arcade release, maybe even after UFGT (though no Poisons making it very far in that tournament doesn’t help us confirm anything there.)
Either way, between this and a couple other glitchy issues, I’m under the impression that all we did was pay to beta test this game for the disc release.
This is quite hilarious actually. Combofiend does this numerous times in the video that was just released on the official Capcom fighters youtube channel giving an overview of Poison. You can tell he wants a fireball and instead does a jump and button.
The game currently feels unfinished to me and rushed out the door so it could be at EVO. :\
HAHA oh my god, thank you for figuring this out.
I really thought that my execution was shit, or that my stick was breaking. I’m seriously cracking up right now that I know, thanks a ton man.
Actually curious, why do you all tiger knee the fireballs with other characters? The only reason that I knew that you could do this was because of rapid seismos, what benefit does it give fireball moves? It seems much harder to do than just a normal QCF, and I’ve never once jumped out of throwing a fireball with poison.
I’ve had a similar situation, but sometimes I get her U2 when I do my rekkas.
Didn’t feel like I was doing anything differently.