Undeath to Death not killing undead

[Kalthariam]

Description:

Casting the undeath to death spell upon undead is not killing undead, it's dropping them instead to 1 hp.
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Steps to reproduce:

Cast Undeath to Death of Wraiths and undead ghosts under the Eastern Ruins of the UD.
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Expected result:

ON a failed save, undead die.
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Actual result:

On a failed save, undead instead take a large amount of positive energy damage, but are left alive (Assuming it's dropping them to 1 hp as the values vary, but they are all dropped to "near dead" and a 5 damage air elemental aura tick kills them.
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Misc Notes:

I am a healer cleric, and slotting Undeath to death in every slot available to me (No applicable meta magics)
This issue applied to every spellslot I cast it from.
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Which Server(s)?

Underdark (Have not tested it on other servers, as places one can get alot of mileage out of spamming undeath to death are limited)
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Gaming Platform(s)?

PC - Steam

[Kalthariam]

Follow up:

I did a bit of testing on the Surface Server, and got intermitted results.

I used summons this time that didn't have passive damage auras (I'm squishy Q_Q) and tested out undeath to death heavily on the eternal battlefield.

Several of the undead on the eternal battle field map (Surface Server) acted like normal what you'd expect, they fail the save, they just die.

However I noticed for whatever reason Undead with the descriptor of Evil ---- seemed to have the same bugs as the spirits under the eastern ruins (Commonly refered to as spirits in the UD back when it was a popular grinding spot) where the undeath to death doesn't kill them and just does a large amount of damage.

Further it doesn't seem to actually be putting htem at 1 HP, for when I followed up my casts with sacred flame, in some instances the undead survived more than 1 sacred flame. (Level 30 Cleric Casting Sacred Flame positive damage cantrip)

I don't know what is causing the spell to not interact properly, but as my original bug report only had information from the one zone, I thought to go somewhere else and extensively test it (Thank you undeath to death refreshes) on other enemies in their natural habitat.

[Kalopsia]

The damage is likely reduced by immunity or resistance against positive damage, which would be intended behavior.

[Kalthariam]

is the spell not suppose to just.. literally outright kill undead that fail the save?

It doesn't say it does damage, it says it slays the undead. (Within certain HD)

Are other save or die spells prevented from killing by someone having a defensive essence or elemental immunity?

[-XXX-]

Undead creatures are immune to instant death effects. To get around that the spell instead deals positive damage equal to the target's hp.
If they happen to have positive DR/DI, they end up with a handful of hp left after the spell resloves.

[Kalthariam]

I see.. well that's rather frustrating from the perspective of someone whom rather enjoys rapidly casting undeath to death on clusters of undead, it's already hard limited by HD (Which means usually only 1-2 mobs get effected by it at a time) but also they just.. don't die when they fail the save? Rather frustrating since that means they will snap aggro off others and come charge you down, when they should be dead.

Is there no way to increase the damage to deal over it's maximum HP in damage? Like 120% for example to likely offset any immunities or resistances?