Is Shackler's Mastery working as intended?

With 5 Razorfoot Griffons in the team, I do the same damage (all of my damage comes from Acrobatics) with Shackler’s Mastery, that should add 20% damage for each of the Griffons, or Grim Aura, that adds 20% of Speed as damage.

Also, at high enemy levels, some attacks randomly do 0 damage (1 with Rage) then start doing the normal 40 million.

Edit: Wait, could it be that Deafening Roar doesn’t stack? I tested it and seemed to stack, but there’s so much variance in the damage that who knows.

Edit 2: Adding Grim Aura or a sixth Deafening Roar on top of the Mastery doesn’t increase the damage at all. It seems 41M is the maximum I can do for some reason.

Seems to be working correctly, and Deafening Roar does stack. It could be some other trait or effect that is causing the damage to appear to be capped.

From what I’ve seen, additional damaage effects don’t seem to trigger if the attack would land for 0. I noticed this using Calamity. Either hit for 0 or a few hundred k. Buff attack or reduce defense on a target I hit for 0, attack again, and boom! Oneshot.