The bug

When the player has weakness strong enough to reduce your damage to zero, your attacks have no effect at all – no damage hurt animation/knockback. However this is not the case for mobs. When mobs with weakness strong enough to reduce their damage to zero attack you, you take knockback and hitstun.

How to reproduce

1. /effect @s weakness 1000 100 true
2. Try to punch a mob ( No knockback/hurt sound/animation)
3. /summon husk
4. /effect @e[type=husk] weakness 1000 100 true
5. Let the husk attack you ( You take knockback/hurt sound/animation)