Socketed Gems get a 90% Mana Multiplier - not being fully applied

Issue:
I have a hat with "Socketed Gems get a 90% Mana Multiplier" but they are not getting the full multiplier.

Status Quo:
I'm running a mana guardian auramancer with 36% reduced mana reservation on the tree.
I am wearing a Memory Vault, resulting in 26% reduced mana reservation.

My tooltip (and actual) mana reservation for 35% aura (vitality) is 26% (25.9% rounded up).


Testing:
I swapped to a Memory Vault with the corruption mod "Socketed Gems get a 90% Mana Multiplier" (see item stats below), and aura mana cost went to 25%. That's a mana multiplier of maybe 96%, not 90%.

I thought maybe there was something odd in when or how the mana multiplier is being calculated so I tried putting a lvl 3 Enlighten (92% mana multiplier) into a normal Memory Vault with the same aura and the tooltip and Vitality had a tooltip (and actual) mana reservation of 24%.
That's a larger effect with a .92 than a .90.

Can you clarify if there are any errors in my math, and why a .90 mana multiplier would have less of an impact than a .92 mana multiplier?


Rarity: Unique
Memory Vault
Praetor Crown
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Armour: 161
Energy Shield: 197 (augmented)
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Requirements:
Level: 68
Str: 62
Int: 138
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Sockets: R-B-B-B
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Item Level: 75
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+2 to Level of Socketed Duration Gems
Socketed Skill Gems get a 90% Mana Multiplier
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+149 to maximum Energy Shield
+177 to maximum Mana
30% increased Mana Regeneration Rate
+20% to Fire Resistance
10% increased Mana Reserved
Gain Armour equal to your Reserved Mana
--------
Your memories are a bridge to the past.
Let no one cross.
--------
Corrupted

Has anyone been able to test the helmet mana multiplier in a normal setting? I only have that one character in HC incursion...

Seems others already reported;
https://www.pathofexile.com/forum/view-thread/2167803/page/1#p15697646

Feel free to move our delete this thread.

I see you found the other threads, just to point out all your numbers are consistent with my running theory that the mana reservation is actually 95% (from the old chest corrupt):

base mana cost = 35
mana cost multiplier = 95%
reduced mana reserved = 26%

First the game calculated mana cost by rounding down, resulting in 35*.95 = 33.25 -> 33.
Then the game factors in reduced mana reserved by rounding up, resulting in 33 * .74 = 24.42 -> 25.

However with enlighten we get
35*.92 .= 32.2 -> 32
32*.74 = 23.68 -> 24