Knockback does not get multiplied when you have the knockback enchantment on an item or if you are sprinting.

Knockback done on players will not get multiplied when damage is done with the knockback enchantment or attacking while sprinting.

Steps to reproduce:
Sprint while attacking a player or enchant a sword with knockback and then attack a player. The knockback should be the same as hitting a player without sprint or the enchantment.

Video
https://streamable.com/a5rcu