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.