Again, why would you mash F/D while doing cr.HP? You're purposely fishing for stuff in that regard. By mashing F/D, you're actually getting the DP motion randomly, combined with neg edge on HP when releasing it. It's a random chance thing and you shouldn't be mashing dir during hits anyway. Although I see something in Vans Rugal that I'll try and introduce.
I see now. Especially when playing with a keyboard, you're not actually doing a HCB. You're stringing your fingers across like a piano, F D B. No DF/DB. Unfortunately, the only sure fire way I can make sure that you don't get accidental fireballs, is by coding it as a true HCB motion(which it is currently). "F DF D DB B." This means you have to press DF-DB to get the move, so holding forward during cl.HP > Tatsu registers fine. This is why you should use a controller. Still, Rugal and Terry don't have moves that conflict with HCB/F. By you holding forward, which despite what you think doesn't help connect, you're performing F D B when trying to do the Tatsu. If you're not close enough for cl.HP to hit upon landing from a jump attack, holding forward isn't going to make it connect. Not truly, it won't be a real combo, just something you're doing in practice.
Let me give you an example with Vans' Rugal. He can cancel from his cr.HK. Try cancelling from cr.HK > God Press with meter. You'll get the super variant every time, even though you're just trying to do the normal G.Press. Because of the shortcut method for HCB/F, when cancelling from cr.HK, I have to go to F from D after tripping p2. This will register as D > F. Then when I go back from F to D >B, this is all registered as D > (F > D > B) so I get the super instead.
Akuma has a lot of commands that conflict with the shortcut method for HCB/F, if you're holding the dir to perform it. If you're holding F, then try to do D > B, it's gonna be F > D > B. I can make the window smaller, the timing more strict, but as long as you hold forward, it will always be the case. Since the command is a more complex one(more than Tatsu), I can't make the window too small, or you won't be able to perform it. Try this; In the tinybuffering.txt, there's a set of code that looks like this:
Spoiler, click to toggle visibilty
Change the number 10 to a higher value. This will increase the allowance window of the input. Try 13 first. If you still can't do it, try 17/18. If you still can't perform it, I guess a true HCB just doesn't vibe well with a keyboard. I'll keep looking over some things though. Maybe adding dir holding restrictions like Vans has done will help. I'll have to experiment some more later.