Here's the situation:

I had a plan to make an archmage and then complete the game via ultimate chaos god ending.
Candle Gray Elven Wizard was selected for this purpose.
Things went well: did all the right quests got all the important items, maxed out char level.

Now I have level 50, C- crowned chaotic champion, with 940 HP, 1470 PP, blessed spellbook of wish - the spell is already learned (230 charges) - great book caster talent has been obtained and it is darknight.
From my calculations based on Ancardia Wiki, bookcasting wish in this situation should cost 2250PP
Base cost of 9000 PP (3x3000 for bookcasting) gets first reduced to 4500 due to great book caster multiplier, which is 1.5 (so 9000x1.5=4500).
Then (or before, it's irrelevant) the darknight reduction is applied - further bringing bookcasting costs down to 50% (2250 PP) of the previous value (4500).
Given that the rough ratio of HP-PP conversion for the purpose of bookcasting is currently about ~1.9 (it's actually 1.9157, but let's dispense with the decimals), I should get this:

Wish cost 2250 = 1470 PP + HP

Since HP counts for almost twice the value of PP, we can assume HP-PP conversion to be: 940*1.9=1786

Thus the final equation:

1470PP + 780PP(from HP) = 2250 <- this is where the wish has been bookcast.

So I should have 0 PP and [(940*1.9) - 780] / 1.9 = ~530 HP left after this, right?

Then how is this possible my character has died?
The final score shows -620 HP.

Of course I have checked *everything* else.
No special rooms, no corruptions, no items that modify attributes, save for red trident, GoGS, elemental gauntlets and orb of mana.
Typical archmage setup that I have successfully used on the previous one.
No intrinsics that impact casting or pp/hp values, no cursing or dooming, etc.



My current conclusion is this: -620(940) HP would suggest that over 3000 PP were drained from HP after 1500 was drained normally from PP.
This would in turn mean that the real bookcast cost is still 4500 so clearly one of the two reducers did not work (as they typically stack).
I've decided to wait 5 days, to be sure darknight has passed. After bookcasting wish again, it turns out the negative HP is *again* 620.
This means that darknight cast cost reduction was not applied when it should have been, though I can't be 100% certain this is the core of the problem.

I have copied the savefile before this occurred because I'm trying to understand what happened. Any suggestions?
I know for sure that I had almost exactly the same HP and PP values when I made an L+ archmage in the very same pre20.
Calculations were done properly then and I had no surprises - each initial bookcasting of wish left me with 400-500 HP back then, which further supports the fact that the cost was indeed 2250 (great book caster + silvernight)

But he situation here is different for some reason.
Is there something I have overlooked?
Some C- penalties perhaps?
Maybe there are some bonuses for L+ characters that even further reduce casting costs which I did not realize existed?

Perhaps the simple explanation is that C- character in question has just discovered a bug where darknight does not benefit the PC as advertised?