Wouldn't that would mean each individual soul is a separate data-point the game has to track? Rather than tracking how many of each item, it would have to record what each soul contains. Would perhaps be more likely for it to instead track whatever rng value was used when you open a soul, to re-use if the summoning gets cancelled for whatever reason?
I've not experienced it myself yet, but I'm sure people have been able to pull new characters from old souls. It's possible everyone who got a werewolf before the Halloween event started earned those souls at the same time?