Negative Energy Absorption items

I was just running through King's Forest and fought a rare necromancer when this problem came up:

I'm wearing a Silver Flame Pendant and Death's Door; the necro started spamming energy drain, ran the pendant out of charges, and I started getting negative levels, without Death's Door running out of charges.

Is there some bug when using two Negative Energy Absorption items? Or is Death's Door not working right?

Try flipping it. Wear only death's door and see if it uses any charges.

Hmmm, Death's Door by itself seems to work. Must be a problem with wearing two items with the same effect.