The abilities UI isn't going to display the current value of Iron Skin's health if you absorb damage during the invulnerability duration or if health is lost after the invulnerability ends. The modified health value that's displayed in the abilities UI is what Iron Skin's initial health would be at 100% before absorbed damage is factored.

The absorbed damage that's indicated in the HUD next to the health and shield bars is added to Iron Skin's modified health value even if it's not displayed in the abilities UI.

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