minecraft.custom:damage_dealt records incorrect damage

The statistic minecraft.custom:minecraft.damage_dealt will not record how much damage was dealt by an attack, but rather how much damage was subtracted from the target's health. If you damage an enemy at 2 health with an attack that deals 6 damage, the statistic will only increment by 20 rather than 60.

 

To recreate, simply create a scoreboard objective with the damage_dealt statistic and damage an enemy.