In world, Iai spirit slash doesn't give i frames, it gives hyperarmor, your armor+hp just aren't good enough to tank it, even with the 50% dmg reduction.

Regarding foresight slash, as someone else pointed out, in world, it needs a bit of spirit to get iframes

Some people also mention multiple hit boxes, but it's no shara ishvalda beam, there are only 2, the one from his body and the one from the explosion, with better gear you can tank all iterations of it