MC 1.21.4
NF 21.4.80
Nameless 1.0.5
EnchantmentLevelBreak 1.4

Callus
-Negates damage from: Cactus, Anvils, Hot Floors and Sweet Berry Bushes.
-Fall damage is reduced by 10%.
-Damage taken is reduced by 5%.

Instead of reduced damage taken it's increased damage taken and feels like it's more than 5%.

Piglin Brutes would instakill me with full diamond armor with protection 30 on all armor pieces. So the damage is also ignoring armor.

The damage it reduced was the original damage before going through any kind of reduction (like armor). I've already fixed the issue, it will be in the next patch.