Heartless Angel inconsistent damage

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Date & Time: March 7th, 2018, between 9:00PM and 10:00PM GMT
Frequency: Three times
World name: Zodiark
Character name: Sifrid Extan
Monster name: Kefka (God form)
Class/Level: Warrior Level 70
Party or solo: Full party
Area and coordinates: Sigmascape V4.0 (Savage)
Steps:
1. Be a Warrior main-tanking God form Kefka
2. Reach the first "Heartless Angel" attack from Kefka
3. Die (sometimes)

Detailed description:

Usually, the Heartless Angel attack reduces all players to 1 HP exactly. A couple of times, I got killed by that attack though. I was the only one who died from it.

(Example values follow, but they're approximately correct, I just don't remember the exact numbers)
My HP in Deliverance was 65000 (using food and party buff). Usually Heartless Angel hits me for 64999 damage. Those times that I died, the damage values I saw were around 69k, 79k and 75k (not sure about the last one).

These don't line up with my Maximum HP in Defiance either, which is around 81250, or using Thrill of Battle, 78000.