likelihood is greed severs hit box passes through invulnerable portion of your 6P and hits your character where they're still vulnerable.
chances are, if you're trying to 6P on reaction, you're reacting late, and the greed sever reaches the lower body "hurtbox" before the 6P is active.
edit: just block it, it's hella negative. if you can see it and attempt to 6P, just IB it instead and punish him. depending on resources you have available (especially if you're playing Jam), punishing him for the ground combo is probably a better reward than a 6P anti air combo anyway.