Demacian Justice vs Flash

For some reason, I read this as Jamaican justice. But no. It shouldn't. If all point and click ults are guaranteed their damage, then this is a bug.

No. Point and click abilities cannot be stopped unless the target becomes untargetable or if the ability is a projectile and is intercepted by a Wind Wall or Unbreakable. But it looks like the ability didn't get cast as the ability isn't on cooldown.