i'm gonna go on a limb here and say it's because they don't want a f2p char to be so powerful so they had to nerf her, kakuzu and naruto 6p need to shine after all

the opponents chakra has to be lower than 30 lol that only happens in the 1st round. Shizuka was made to be a 1st move but they completely ruined it... Only time the opponent has less than 30 chakra is during the 1st round and you can't do your mystery then. If the opponents chakra is higher than 30, you just can't interrupt anyone. They ruined the character on purpose cause it's a f2p char

They can activate the mystery if they're on auto and you interrupt them but that only lasts until round 3, on round 4 the opponents chakra is 80, they will only spend 40 if it's naruto 6p, and if it's kakuzu they don't spend anything at all, shizuka could've been a great kakuzu counter and a naruto counter if she has her original mystery, kakuzu couldn't do his mystery that takes way too much health despite it being ''medium'' damage, now you can't do that, cause it doesn't interrupt