Multiplicative enchantment damage bonuses stack additively with critical attack damage bonus.

A custom enchant which increases damage by a percentage per level stacks additively with the critical hit multiplier, rather than multiplicatively.

Below are screenshots showing a custom enchantment which increases sword damage by 20% per level. This benchmark was done with two netherite swords, one enchanted with the enchantment, one unenchanted, against an Iron Golem.

Note when critical attacking with an unenchanted netherite sword, the amount of damage dealt is 8 on a normal hit. With the first level of the enchantment on a netherite sword with a critical hit, 13.6 damage is dealt. 13.6 is a 70% increase (1.7x factor) compared to the unenchanted sword's 8 base damage (without critical hitting), which is an additive increase of 20% onto the 150% multiplier that critical attacking provides.

Also note that the increase between a normal critical hit and an enchanted critical hit in this scenario is a 13.33% increase due to this, when it should be a 20% increase.

The JSON for the custom enchantment (modified Sharpness) is included for troubleshooting purposes.