Additive attributes apply and infinity stack incorrectly on hand swap (1.21.11) #6881

Plugins: MythicMobs + MythicCrucible (latest versions) Server: Paper/Purpur/Spigot 1.21.11 Problem: Additive attributes (especially generic.max_health) stack permanently when swapping hands between vanilla + custom items.

Summary Custom items with ADDITIVE attributes (e.g. extra health hearts) cause permanent stacking when swapping between vanilla and custom items in hands:

Vanilla item (e.g. unenchanted netherite sword) in mainhand → custom item in offhand → no stacking.

Vanilla enchanted item + custom item → swapping hands (F key) applies additive attributes multiple times.

Stats stack and persist until server relog (death does not reset them).

This only happens with custom items having additive attributes; vanilla attributes work normally.

Steps to reproduce

Create items like this:

IcyStar:
  Id: DIAMOND
  Hide:
    - ATTRIBUTES
  Stats:
    - DAMAGE_REDUCTION 0.1 MULTIPLY
    - HEALTH 2 ADDITIVE
    - HEALTH_REGENERATION 1 ADDITIVE
    - FIRE_RESISTANCE 0.1 ADDITIVE
    - ARMOR_FREEZE 1 ADDITIVE

Take item to hand and on second hand one of testing swords (enchanted and simple)
Start to swap custom item with one sword
Same with other and see the difference.
Relog to reset stacked stats.

This issue was marked in the Discord chat by different server so it's global problem.

Video: [Legacy attachment omitted]

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