This is actually a really annoying input overlap that could be easily solved. It bothers me and I wish Capcom would fix it. I’ve tweeted Capcom Fighters, Ono, and ComboFiend about it and I’ve posted about it before, but they’ve never had a response.
I’m going to explain from the ground up.
First, you have to know that each Special Move has a “priority” for the input. This is what lets the game determine which Special Move to execute if you input overlapping direcitonal inputs for Special Moves that use the same attack button.
Second, for when you want Stomp but accidentally get Slash. You ARE hitting “F > D > D+F”, BUT, you’re transitioning from either Back or Down-Back at the start of the input. Starting with Back or Down-Back causes the game to assume you want the quarter-circle-forward input for Slash because it sees you press either a) “B > F [ignored for leniency] > D > DF” or b) “DB > F [ignored for leniency] > D > DF”.
[*]Finally, Slash has a HIGHER Special Move input priority than Stomp, which is why you get Slash.
This is a really annoying issue with Oni for me because if they just made Stomp have higher priority than Slash, this would never happen. I actually had Razor (who makes the [Avenue Brawler mod](Avenue Brawler Mod Manager change the Special Move input priorities and give Stomp a higher priority for me once – it fixed the issue with no negative side affects at all. This is because any time you overlap both inputs, it’s safe to assume Stomp is what the player wanted, since you would never double over F and back to D+F when doing Slash, you’d stop on either D+F or F and never go back.
So Capcom really should make Stomp the higher priority move and Slash the lower priority move. I have no idea why Slash was ever made higher priority than Stomp, but it is annoying and it messes me up all the time, especially after SRK > FADC. I do not want to MK Slash after an SRK FADC, lol… I want to STOMP! >_<
Anyway, since this probably won’t ever be fixed, the way to work around it is to LET GO of back or down-back for a split second and let the stick return to neutral before inputting your “F > D > D+F” for Stomp. That way, the game doesn’t read a Back or a Down-Back input and won’t assume that you want Slash. It’s stupid that you even have to do that, but you do.