You should think of it like 2 gunmen in a shootout. If they both fire their shot at the same time just because Person A's hit first doesn't make person B's disappear at the same time. In the same sense the attack is a blast that occurred just as ifrit died aka they both go off at the same time it's not something you can really prevent unless you manage to kill him before said blast goes off. It's not really a glitch or anything the player in question just wasn't able to take down Ifrit before the blast went off, since it is possible to negate the cracks if you can kill him before they are ready to initiate.