Bonestorm Debuff Damage Bugged

I've done some testing with Sunder+Bonestorm debuff, and it seems like the flat damage from the Bonestorm Debuff isn't being included in the whole damage calculation. Below is how I believe the damage calculation is working, trying to reverse engineer the math on the damage. But from my testing, Bonestorm definitely isn't including all the various multipliers below, so debuffing an enemy with Bonestorm increases my damage FAR less than I'd expect.

Things that modify the Bonestorm debuff flat damage:
1. The % attack multiplier on the attack. For example, a level 20 Sunder's shockwave attack multiplier is 531%.
2. Multipliers on the attack from support gems, Rage, etc.
3. Critical damage multiplier on crits

Things that DO NOT modify the Bonestorm debuff damage:
1. % Increased Attack/Melee damage from passive tree etc.
2. Any damage Gained, for example from Infernal Cry or Ming's Heart rings, doesn't seem to take into account the flat damage debuff from Bonestorm. So no additional damage is Gained from Infernal Cry if the Bonestorm debuff is on enemy, vs off enemy.

I had all 5 of those separate multipliers at very different %s to help figure out which ones are being included in the damage calculation, and which ones aren't, except my % Increased Attack/Melee Damage from passive tree vs the attack multiplier on the attack were about the same. So it's possible that the attack multiplier is NOT affecting the Bonestorm debuff flat dmg, but the % Increased Attack/Melee Damage from the passive tree IS affecting the Bonestorm debuff flat dmg.

Regardless, 2+ of these various multipliers do not seem to be taking the Bonestorm flat dmg into account.

Thanks.