The issue:
Holding a weapon with knockback and using the /damage command and specifying the source player inflicts knockback, but not the knockback from your enchantment.
Reproduction Steps:
1) Summon an example entity: /summon creeper ~ ~ ~ {Tags:["test"]}
2) Give yourself a sword with high knockback to ensure this behavior: /give @s iron_sword{Enchantments:[
{id:"minecraft:knockback",lvl:10s}
]}
3) Use the damage command on the creeper: /execute as @e[type=creeper,tag=test] run damage @s 1 generic by @p
4) Take note as the creeper takes small amount of knockback, as if it was attacked without an item enchanted by knockback
What is happening:
The creeper takes small knockback as if it was attacked with an item that does not have the knockback enchantment.
What should be happening:
The creeper would take the correct amount of knockback corresponding to that of the knockback enchantment level on the item the player is holding.
Seems like using this method but with a fire aspect weapon does not trigger the fire aspect either.