According to the game context, melee attacking in weakness I with bare hand has 0.3 damage. Therefore, killing a pillager or a vindicator needs 24÷0.3=80 strikes in theory, but actually it needs 81 strikes.

And may be the bug lies in living things. For example, the skeleton and creeper are same in health, but different strike times in Weakness II using wooden spade. So does cow and pig, pillager and evoker.

There are 26 mobs in all that have damage modification. If the damage is lower than 1 but higher than 0, it will then be modified to 1. The 26 mobs are: Iron Golems, Snow Golems, Withers, Ender Dragons, Frogs, Allays, Mooshrooms, Bats, Chickens, Parrots, Pigs, Turtles, Villagers, Wandering Traders, Goats, Axolotls, Cats, Ocelots, Bees, Foxes, Llamas, Ghasts, Blazes, Creepers, Magma Cubes, and Breeze.