Deathbringer Saurfang

Deathbringer Saurfang when stolen by Reverberation or other means on a warlock with the death rattle costs health instead of mana does not work with the Felstring Harp

I will copy my explanation from another thread:

Just to elaborate: When you play Deathbringer Saurfang for health, you actually pay the health but you do not take damage. Those are two different things now. They changed this in a recent patch (26.0 I think).

Here a link for more information:
https://us.forums.blizzard.com/en/hearthstone/t/they-are-finally-changing-itcosts-wont-be-damage-anymore/104841

In your case: the weapon only prevents you from taking damage. But you are paying health for playing that minion.

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