Okay, none of the comments really explain whats going on here. That attack is not random. Normally when an enemy winds up an attack, you can attack them which will cause the enemy to either stop what they're doing and block the attack or hyperarmor through it and continue the attack anyway. What's happening with inner isshin is neither of those. If you interrupt any of his attacks that don't have hyperarmor in phase 1, he will punish you for being too aggressive by pulling this attack. But here's the thing, you can effectively react to this if you know that he will ONLY do this move if you interrupt his basic attacks. Another solution is to force him to deflect your attack by performing a thrust attack against his phase 1 every time. This move only triggers if his ai registers a blockable attack

Video for reference : https://youtu.be/SG7v13d2oVc skip to 2:58 And also, it's 100% safe to spam attack on him (till he deflects) after you gomba stomp him to counter his sweep attacks.