Cataclysm has a config-based damage cap system that sets a limit on the amount of damage certain bosses can take. When paired with Apothic Attributes, this cap is halved. Tested on minimal instance with only Cataclysm, AA, and dependencies. Present on versions 1.21.1 and 1.20.1.

1.21.1 logs: https://mclo.gs/xsSXmkc
1.20.1 logs: https://mclo.gs/fO1QTxN

Ignis class: https://github.com/lender544/new1.20.1/blob/master/src/main/java/com/github/L_Ender/cataclysm/entity/AnimationMonster/BossMonsters/Ignis_Entity.java

Parent class:
https://github.com/lender544/new1.20.1/blob/master/src/main/java/com/github/L_Ender/cataclysm/entity/AnimationMonster/BossMonsters/LLibrary_Boss_Monster.java

Cataclysm is capping damage before damage reduction is applied (effectively capping the player's attack damage), not the amount of damage the entity can receive from a single hit. Since apoth's armor calculations make armor with 0 toughness stronger than vanilla, that's where the delta comes from.

Ideally Cataclysm would apply a post-damage-reduction clamp, rather than a pre-reduction clamp; The events to do this are the LivingHurtEvent (1.20) and the LivingDamageEvent.Pre (1.21).

The alternatives are to either strip the armor from that entity, change the Apoth armor formulas, or get yourself some armor pen.