I've experienced not being able to use F-specials out of hitstun that aren't once-per-airtime resources - Zelda's and Bjr's, possibly others. Not sure of the cause, but it usually occurs 0-2 times per 2h session. I have tried inputting additional times from drift after exiting tumble to be sure I didn't just buffer it too early, but it's def a lockout. Can't recall if inputting DJ or AD fixes this or not, will try test next time I encounter it.