Heartpiercer negates both, main armor (base armor value + armor gained through agi) and other armor bonuses (green + armor). But the HUD only shows your main armor being negated.

For example if you have 10 +10 armor, your HUD shows 0 +10 armor when hit by Heartpiercer, instead of showing just 0 .

The green +10 should not be there on the bottom part