Parry chance is calculated before block, so if the attack is parried, it doesn't check against block, and since parrying is all or nothing, no damage popup is displayed.
As for the Olivia battle impact plugin, I can't tell why it would do that, you'd think it'd be either one way, or the other, but not randomly choose between the two. I don't have the plugin myself, however, so I can't look into this problem very well.